ios: __builtin_clz check zero

ijkmedia/ijksdl/ffmpeg/ijksdl_vout_overlay_ffmpeg.c

@@ -156,7 +156,9 @@ SDL_VoutOverlay *SDL_VoutFFmpeg_CreateOverlay(int width, int height, Uint32 form
         buf_width = IJKALIGN(width, 16); // 1 bytes per pixel for Y-plane
 #elif defined(__APPLE__)
         // 2^n align for width
-        buf_width  = 1 << (sizeof(int) * 8 - __builtin_clz(width));
+        buf_width = width;
+        if (width > 0)
+            buf_width = 1 << (sizeof(int) * 8 - __builtin_clz(width));
 #else
         buf_width = IJKALIGN(width, 16); // unknown platform
 #endif