avformat/dxa: Adjust order of operations around block align

Fixes: 51896/clusterfuzz-testcase-minimized-ffmpeg_dem_DXA_fuzzer-5730576523198464
Fixes: signed integer overflow: 2147483566 + 82 cannot be represented in type 'int'

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/dxa.c

@@ -123,7 +123,7 @@ static int dxa_read_header(AVFormatContext *s)
         if(ast->codecpar->block_align) {
             if (c->bpc > INT_MAX - ast->codecpar->block_align + 1)
                 return AVERROR_INVALIDDATA;
-            c->bpc = ((c->bpc + ast->codecpar->block_align - 1) / ast->codecpar->block_align) * ast->codecpar->block_align;
+            c->bpc = ((c->bpc - 1 + ast->codecpar->block_align) / ast->codecpar->block_align) * ast->codecpar->block_align;
         }
         c->bytes_left = fsize;
         c->wavpos = avio_tell(pb);