Fix GError handling

Several users have reported segfaults when starting up profanity which
has OMEMO support, but OMEMO is not set up yet.

@StefanKropp has been able to reproduce this and tracked it down to
`_load_identity()` calling `omemo_known_devices_keyfile_save()`.
The latter then calls `save_keyfile()` which calls
`g_key_file_save_to_file()`. This can then fail if one of the first two
strings is NULL and won't set the `error` on return. In its error handling
`save_keyfile()` unconditionally dereferences `error` which leads to the
segfault.

Fix this and also go through the entire codebase and verify that the usage
of `GError` is done correctly.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

src/config/cafile.c

@@ -70,10 +70,10 @@ cafile_add(const TLSCertificate* cert)
     auto_gchar gchar* contents = NULL;
     auto_gchar gchar* new_contents = NULL;
     gsize length;
-    GError* glib_error = NULL;
+    auto_gerror GError* glib_error = NULL;
     if (g_file_test(cafile, G_FILE_TEST_EXISTS)) {
         if (!g_file_get_contents(cafile, &contents, &length, &glib_error)) {
-            log_error("[CAfile] could not read from %s: %s", cafile, glib_error ? glib_error->message : "No GLib error given");
+            log_error("[CAfile] could not read from %s: %s", cafile, PROF_GERROR_MESSAGE(glib_error));
             return;
         }
         if (strstr(contents, cert->fingerprint)) {
@@ -84,7 +84,7 @@ cafile_add(const TLSCertificate* cert)
     const char* header = "# Profanity CAfile\n# DO NOT EDIT - this file is automatically generated";
     new_contents = g_strdup_printf("%s\n\n# %s\n%s", contents ? contents : header, cert->fingerprint, cert->pem);
     if (!g_file_set_contents(cafile, new_contents, -1, &glib_error))
-        log_error("[CAfile] could not write to %s: %s", cafile, glib_error ? glib_error->message : "No GLib error given");
+        log_error("[CAfile] could not write to %s: %s", cafile, PROF_GERROR_MESSAGE(glib_error));
 }
 
 gchar*

src/config/preferences.c

@@ -89,7 +89,7 @@ _prefs_load(void)
     log_maxsize = g_key_file_get_integer(prefs, PREF_GROUP_LOGGING, "maxsize", &err);
     if (err) {
         log_maxsize = 0;
-        g_error_free(err);
+        PROF_GERROR_FREE(err);
     }
 
     // move pre 0.5.0 autoaway.time to autoaway.awaytime