xkbcomp: Don't explode on invalid virtual modifiers

testcase: 'virtualModifiers=LevelThreC'

Signed-off-by: Daniel Stone <daniels@collabora.com>

src/xkbcomp/expr.c

@@ -101,6 +101,8 @@ LookupModMask(struct xkb_context *ctx, const void *priv, xkb_atom_t field,
         return false;
 
     str = xkb_atom_text(ctx, field);
+    if (!str)
+        return false;
 
     if (istreq(str, "all")) {
         *val_rtrn  = MOD_REAL_MASK_ALL;