avcodec/flac_parser: Do not lose header count in find_headers_search()

Fixes: Timeout
Fixes: out of array access
Fixes: 20274/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLAC_fuzzer-5649631988154368
Fixes: 19275/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FLAC_fuzzer-5757535722405888

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

libavcodec/flac_parser.c

@@ -208,16 +208,20 @@ static int find_headers_search(FLACParseContext *fpc, uint8_t *buf,
     uint32_t x;
 
     for (i = 0; i < mod_offset; i++) {
-        if ((AV_RB16(buf + i) & 0xFFFE) == 0xFFF8)
-            size = find_headers_search_validate(fpc, search_start + i);
+        if ((AV_RB16(buf + i) & 0xFFFE) == 0xFFF8) {
+            int ret = find_headers_search_validate(fpc, search_start + i);
+            size = FFMAX(size, ret);
+        }
     }
 
     for (; i < buf_size - 1; i += 4) {
         x = AV_RN32(buf + i);
         if (((x & ~(x + 0x01010101)) & 0x80808080)) {
             for (j = 0; j < 4; j++) {
-                if ((AV_RB16(buf + i + j) & 0xFFFE) == 0xFFF8)
-                    size = find_headers_search_validate(fpc, search_start + i + j);
+                if ((AV_RB16(buf + i + j) & 0xFFFE) == 0xFFF8) {
+                    int ret = find_headers_search_validate(fpc, search_start + i + j);
+                    size = FFMAX(size, ret);
+                }
             }
         }
     }