lavu/intmath.h: Fix UB in ff_ctz_c() and ff_ctzll_c()

Found by value analysis

libavutil/intmath.h

@@ -119,7 +119,7 @@ static av_always_inline av_const int ff_ctz_c(int v)
         0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
         31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
     };
-    return debruijn_ctz32[(uint32_t)((v & -v) * 0x077CB531U) >> 27];
+    return debruijn_ctz32[(uint32_t)((v & -(uint32_t)v) * 0x077CB531U) >> 27];
 }
 #endif
 
@@ -135,7 +135,7 @@ static av_always_inline av_const int ff_ctzll_c(long long v)
         63, 52, 6, 26, 37, 40, 33, 47, 61, 45, 43, 21, 23, 58, 17, 10,
         51, 25, 36, 32, 60, 20, 57, 16, 50, 31, 19, 15, 30, 14, 13, 12
     };
-    return debruijn_ctz64[(uint64_t)((v & -v) * 0x022FDD63CC95386DU) >> 58];
+    return debruijn_ctz64[(uint64_t)((v & -(uint64_t)v) * 0x022FDD63CC95386DU) >> 58];
 }
 #endif