lavu: prevent overflow in av_clip_intp2_c

This fixes ubsan runtime error: signed integer overflow: 8388608 +
2140274688 cannot be represented in type 'int'

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavutil/common.h

@@ -211,7 +211,7 @@ static av_always_inline av_const int32_t av_clipl_int32_c(int64_t a)
  */
 static av_always_inline av_const int av_clip_intp2_c(int a, int p)
 {
-    if ((a + (1 << p)) & ~((2 << p) - 1))
+    if (((unsigned)a + (1 << p)) & ~((2 << p) - 1))
         return (a >> 31) ^ ((1 << p) - 1);
     else
         return a;