xfree86: ddc: safety check for NULL pointer

Even though it *should* never be actually hit, it's still safer
to check for NULL instead of letting us crash with segfault.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

hw/xfree86/ddc/ddc.c

@@ -45,6 +45,9 @@ find_start(unsigned int *ptr)
     unsigned int comp[9], test[9];
     int i, j;
 
+    if (!ptr)
+        return -1;
+
     for (i = 0; i < 9; i++) {
         comp[i] = *(ptr++);
         test[i] = 1;