Harden compiler flags, simplify CWE-134 script, fix bugs found by new warnings

configure.ac:
- Replace basic -Wformat/-Wformat-nonliteral with -Wformat=2
- Add -Wextra, -Wnull-dereference, -Wpointer-arith, -Wimplicit-function-declaration
- Add -fstack-protector-strong, -fno-common, -D_FORTIFY_SOURCE=2
- Add GCC-specific flags via AC_COMPILE_IFELSE: -Wlogical-op, -Wduplicated-cond,
  -Wduplicated-branches, -Wstringop-overflow
- Add linker hardening via AC_LINK_IFELSE: -Wl,-z,relro -Wl,-z,now
- Suppress noisy -Wextra sub-warnings: -Wno-unused-parameter,
  -Wno-missing-field-initializers, -Wno-sign-compare, -Wno-cast-function-type
- Remove AM_CFLAGS/CFLAGS duplication line

check-cwe134.sh:
- Reduce from 5 checks to 2 (checks 1-3 are now redundant with -Wformat=2)
- Check 1: verify known wrappers have G_GNUC_PRINTF attribute
- Check 2: auto-detect unannotated variadic printf-like functions

Bug fixes found by -Wduplicated-branches:
- chatlog.c: non-MUCPM redact path passed resourcepart instead of NULL
- rosterwin.c: two instances of if/else with identical branches in roster count
- omemo.c: redundant else-if branch in omemo_automatic_start

Other fixes for new warnings:
- console.c: pointer compared to integer 0 instead of NULL (2 instances)
- vcard.c: NULL guard for filename before g_file_set_contents
- files.c: refactor to early return, eliminating NULL logfile path
- database.c: const-correctness for type, query, sort variables
- form.c/xmpp.h: const-correctness for form_set_value parameter
- muc.c/muc.h: remove meaningless top-level const on return type
- common.c: const-correctness for URL string literal
- xmpp/omemo.c: scope block for declarations after goto, move from decl
  before goto, replace goto with direct return
- http_common.h: add G_GNUC_PRINTF attributes for http_print_transfer*
- test_common.c: add currb NULL check to silence -Wnull-dereference

src/xmpp/omemo.c

@@ -353,6 +353,7 @@ omemo_receive_message(xmpp_stanza_t* const stanza, gboolean* trusted)
 {
     char* plaintext = NULL;
     const char* type = xmpp_stanza_get_type(stanza);
+    const char* from = xmpp_stanza_get_from(stanza);
     GList* keys = NULL;
     unsigned char* iv_raw = NULL;
     unsigned char* payload_raw = NULL;
@@ -434,8 +435,6 @@ omemo_receive_message(xmpp_stanza_t* const stanza, gboolean* trusted)
         keys = g_list_append(keys, key);
     }
 
-    const char* from = xmpp_stanza_get_from(stanza);
-
     plaintext = omemo_on_message_recv(from, sid, iv_raw, iv_len,
                                       keys, payload_raw, payload_len,
                                       g_strcmp0(type, STANZA_TYPE_GROUPCHAT) == 0, trusted);
@@ -471,7 +470,8 @@ _omemo_receive_devicelist(xmpp_stanza_t* const stanza, void* const userdata)
 
         const char* code = xmpp_stanza_get_attribute(error, "code");
         if (g_strcmp0(code, "404") == 0) {
-            goto out;
+            omemo_set_device_list(from, NULL);
+            return 1;
         }
     }
 
@@ -520,22 +520,25 @@ _omemo_receive_devicelist(xmpp_stanza_t* const stanza, void* const userdata)
         goto out;
     }
 
-    xmpp_stanza_t* list = xmpp_stanza_get_child_by_ns(item, STANZA_NS_OMEMO);
-    if (!list) {
-        return 1;
-    }
-
-    xmpp_stanza_t* device;
-    for (device = xmpp_stanza_get_children(list); device != NULL; device = xmpp_stanza_get_next(device)) {
-        if (g_strcmp0(xmpp_stanza_get_name(device), "device") != 0) {
-            continue;
+    /* New scope to keep declarations after goto out above */
+    {
+        xmpp_stanza_t* list = xmpp_stanza_get_child_by_ns(item, STANZA_NS_OMEMO);
+        if (!list) {
+            return 1;
         }
 
-        const char* id = xmpp_stanza_get_id(device);
-        if (id != NULL) {
-            device_list = g_list_append(device_list, GINT_TO_POINTER(strtoul(id, NULL, 10)));
-        } else {
-            log_error("[OMEMO] received device without ID");
+        xmpp_stanza_t* device;
+        for (device = xmpp_stanza_get_children(list); device != NULL; device = xmpp_stanza_get_next(device)) {
+            if (g_strcmp0(xmpp_stanza_get_name(device), "device") != 0) {
+                continue;
+            }
+
+            const char* id = xmpp_stanza_get_id(device);
+            if (id != NULL) {
+                device_list = g_list_append(device_list, GINT_TO_POINTER(strtoul(id, NULL, 10)));
+            } else {
+                log_error("[OMEMO] received device without ID");
+            }
         }
     }