gbytes: Add an assertion to placate static analysis

Otherwise `scan-build` thinks it’s possible for the `GBytes` to be
double-freed, which would indeed happen if `try_steal_and_unref()` were
to return `NULL` on this branch.

It’s not actually possible for it to return `NULL` here though, as if
`bytes->data` were `NULL`, the function would have already returned
higher up.

Fixes this `scan-build` failure:
https://gitlab.gnome.org/GNOME/glib/-/jobs/4359929

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

glib/gbytes.c

@@ -490,6 +490,7 @@ try_steal_and_unref (GBytes         *bytes,
     {
       *size = bytes->size;
       result = (gpointer)bytes->data;
+      g_assert (result != NULL);  /* otherwise the case of @bytes being freed can’t be distinguished */
       g_free (bytes);
       return result;
     }