avcodec/exr: fix memset first arg in reverse_lut()

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/exr.c

@@ -322,7 +322,7 @@ static uint16_t reverse_lut(const uint8_t *bitmap, uint16_t *lut)
 
     i = k - 1;
 
-    memset(lut + k, 0, (USHORT_RANGE - k) * 2);
+    memset(lut + k * 2, 0, (USHORT_RANGE - k) * 2);
 
     return i;
 }