comctl32/tests: Avoid uninitialized variable warning in GetWidthBytes().

dlls/comctl32/tests/imagelist.c

@@ -753,7 +753,7 @@ static INT DIB_GetWidthBytes( int width, int bpp )
         default:
             trace("Unknown depth %d, please report.\n", bpp );
             assert(0);
-            break;
+            return -1;
     }
     return 4 * words;
 }