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>

.github/workflows/main.yml

@@ -35,10 +35,10 @@ jobs:
       # if this check fails, you have to update the number of auto types known and the list of auto types in the check below
       - name: Check auto types are up-to-date
         run: |
-          [[ "$(find src -type f -name '*.[ch]' -exec awk '/^#define auto_[\W]*/ {print $2}' '{}' \; | sort -u | wc -l)" == "8" ]] || exit -1
+          [[ "$(find src -type f -name '*.[ch]' -exec awk '/^#define auto_[\W]*/ {print $2}' '{}' \; | sort -u | wc -l)" == "9" ]] || exit -1
       - name: Check auto types are initialized
         run: |
-          grep -P 'auto_(char|gchar|gcharv|guchar|jid|sqlite|gfd|FILE)[\w *]*;$' -r src && exit -1 || true
+          grep -P 'auto_(char|gchar|gcharv|gerror|guchar|jid|sqlite|gfd|FILE)[\w *]*;$' -r src && exit -1 || true
       - name: Run clang-format
         uses: jidicula/clang-format-action@v4.11.0
         with: