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/ui/tray.c

@@ -81,7 +81,7 @@ _get_icons(void)
 
     auto_gchar gchar* icons_dir_s = files_get_config_path(DIR_ICONS);
     icons_dir = g_string_new(icons_dir_s);
-    GError* err = NULL;
+    auto_gerror GError* err = NULL;
 
     if (!g_file_test(icons_dir->str, G_FILE_TEST_IS_DIR)) {
         return;
@@ -109,8 +109,7 @@ _get_icons(void)
         }
         g_string_free(name, TRUE);
     } else {
-        log_error("Unable to open dir: %s", err->message);
-        g_error_free(err);
+        log_error("Unable to open dir: %s", PROF_GERROR_MESSAGE(err));
     }
     g_dir_close(dir);
     g_string_free(icons_dir, TRUE);