glob: let I_NextGlob() return NULL if (glob == NULL)

If a non-existent directory is passed over to I_StartGlob(), e.g. when
moving config files between computers, it will return NULL. If this
return value is then fed into I_NextGlob(), it will trigger a
segmentation fault. Add a check to the latter if (glob == NULL) and
let it return NULL itself in this case.

src/i_glob.c

@@ -319,6 +319,11 @@ const char *I_NextGlob(glob_t *glob)
 {
     const char *result;
 
+    if (glob == NULL)
+    {
+        return NULL;
+    }
+
     // In unsorted mode we just return the filenames as we read
     // them back from the system API.
     if ((glob->flags & GLOB_FLAG_SORTED) == 0)