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/pgp/gpg.c

@@ -186,7 +186,7 @@ p_gpg_on_connect(const char* const barejid)
         auto_gchar gchar* keyid = g_key_file_get_string(pubkeyfile, jid, "keyid", &gerr);
         if (gerr) {
             log_error("Error loading PGP key id for %s", jid);
-            g_error_free(gerr);
+            PROF_GERROR_FREE(gerr);
         } else {
             gpgme_key_t key = NULL;
             error = gpgme_get_key(ctx, keyid, &key, 0);