gfileutils: Add an assertion to help static analysis

This assertion basically mirrors the existing `buffer_size >= 2`
assertion (one implies the other), but scan-build doesn’t seem to be
able to work that out — so give it some help.

Fixes:
```
../../../glib/gfileutils.c: In function ‘g_get_current_dir’:
../../../glib/gfileutils.c:2995:17: warning: potential null pointer dereference [-Wnull-dereference]
 2995 |       buffer[1] = 0;
      |       ~~~~~~~~~~^~~
../../../glib/gfileutils.c:2994:17: warning: potential null pointer dereference [-Wnull-dereference]
 2994 |       buffer[0] = G_DIR_SEPARATOR;
```

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

glib/gfileutils.c

@@ -2991,6 +2991,7 @@ g_get_current_dir (void)
     {
       /* Fallback return value */
       g_assert (buffer_size >= 2);
+      g_assert (buffer != NULL);
       buffer[0] = G_DIR_SEPARATOR;
       buffer[1] = 0;
     }