avutil/mathematics: Do not treat INT64_MIN as positive in av_rescale_rnd

The code expects actual positive numbers and gives completely wrong
results if INT64_MIN is treated as positive
Instead clip it into the valid range that is add 1 and treat it as
negative

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 25e37f5ea9)

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/mathematics.c

@@ -76,8 +76,8 @@ int64_t av_rescale_rnd(int64_t a, int64_t b, int64_t c, enum AVRounding rnd)
         rnd -= AV_ROUND_PASS_MINMAX;
     }
 
-    if (a < 0 && a != INT64_MIN)
-        return -av_rescale_rnd(-a, b, c, rnd ^ ((rnd >> 1) & 1));
+    if (a < 0)
+        return -av_rescale_rnd(-FFMAX(a, -INT64_MAX), b, c, rnd ^ ((rnd >> 1) & 1));
 
     if (rnd == AV_ROUND_NEAR_INF)
         r = c / 2;