fix: null pointer dereference guards (CWE-476)

- cmd_funcs.c: check strtok() return before use in cmd_process_input
- cmd_funcs.c: check fdopen() return before use in cmd_sendfile
- omemo.c: init fingerprint_len=0 and check _omemo_fingerprint_decode()
  return in omemo_is_trusted_identity (CWE-457/CWE-690)
- omemo.c: early NULL return in _omemo_fingerprint_decode() for NULL input

Found via GCC -fanalyzer. Not covered by upstream.

src/command/cmd_funcs.c

@@ -210,6 +210,9 @@ cmd_process_input(ProfWin* window, char* inp)
     if (inp[0] == '/') {
         auto_char char* inp_cpy = strdup(inp);
         char* command = strtok(inp_cpy, " ");
+        if (!command) {
+            return TRUE;
+        }
         char* question_mark = strchr(command, '?');
         if (question_mark) {
             *question_mark = '\0';
@@ -4963,6 +4966,10 @@ cmd_sendfile(ProfWin* window, const char* const command, gchar** args)
     }
 
     FILE* fh = fdopen(fd, "rb");
+    if (!fh) {
+        cons_show_error("Failed to open file for reading.");
+        goto out;
+    }
 
     if (omemo_enabled) {
 #ifdef HAVE_OMEMO

src/omemo/omemo.c

@@ -1089,8 +1089,11 @@ omemo_is_trusted_identity(const char* const jid, const char* const fingerprint)
         .device_id = GPOINTER_TO_INT(device_id),
     };
 
-    size_t fingerprint_len;
+    size_t fingerprint_len = 0;
     unsigned char* fingerprint_raw = _omemo_fingerprint_decode(fingerprint, &fingerprint_len);
+    if (!fingerprint_raw) {
+        return FALSE;
+    }
     unsigned char djb_type[] = { '\x05' };
     signal_buffer* buffer = signal_buffer_create(djb_type, 1);
     buffer = signal_buffer_append(buffer, fingerprint_raw, fingerprint_len);
@@ -1149,6 +1152,9 @@ _omemo_fingerprint(ec_public_key* identity, gboolean formatted)
 static unsigned char*
 _omemo_fingerprint_decode(const char* const fingerprint, size_t* len)
 {
+    if (!fingerprint) {
+        return NULL;
+    }
     unsigned char* output = malloc(strlen(fingerprint) / 2 + 1);
 
     int i;