avcodec/exr: fix clearing end of bitmap

Inspired by patch from Martin Vignali.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/exr.c

@@ -768,7 +768,7 @@ static int piz_uncompress(EXRContext *s, const uint8_t *src, int ssize,
     if (min_non_zero <= max_non_zero)
         bytestream2_get_buffer(&gb, td->bitmap + min_non_zero,
                                max_non_zero - min_non_zero + 1);
-    memset(td->bitmap + max_non_zero, 0, BITMAP_SIZE - max_non_zero);
+    memset(td->bitmap + max_non_zero + 1, 0, BITMAP_SIZE - max_non_zero - 1);
 
     maxval = reverse_lut(td->bitmap, td->lut);