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/common.h

@@ -112,12 +112,22 @@ void auto_close_gfd(gint* fd);
 
 void auto_close_FILE(FILE** fd);
 
+void auto_free_gerror(GError** err);
+#define auto_gerror __attribute__((__cleanup__(auto_free_gerror)))
+
 #if defined(__OpenBSD__)
 #define STR_MAYBE_NULL(p) (p) ?: "(null)"
 #else
 #define STR_MAYBE_NULL(p) (p)
 #endif
 
+#define PROF_GERROR_MESSAGE(err) (err) ? (err)->message : "error message missing"
+#define PROF_GERROR_FREE(err)  \
+    do {                       \
+        if (err)               \
+            g_error_free(err); \
+    } while (0)
+
 typedef struct prof_keyfile_t
 {
     gchar* filename;