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/plugins/plugins.c

@@ -214,10 +214,8 @@ plugins_uninstall(const char* const plugin_name)
     g_string_append(target_path, "/");
     g_string_append(target_path, plugin_name);
     GFile* file = g_file_new_for_path(target_path->str);
-    GError* error = NULL;
-    gboolean result = g_file_delete(file, NULL, &error);
+    gboolean result = g_file_delete(file, NULL, NULL);
     g_object_unref(file);
-    g_error_free(error);
     g_string_free(target_path, TRUE);
     return result;
 }