tools/target_dem_fuzzer: Check fmt before dereferencing

Fixes: NULL pointer dereference
Fixes: 44884/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-4656748688965632

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

tools/target_dem_fuzzer.c

@@ -173,7 +173,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
     }
 
     // HLS uses a loop with sleep, we thus must breakout or we timeout
-    if (!strcmp(fmt->name, "hls"))
+    if (fmt && !strcmp(fmt->name, "hls"))
         interrupt_counter &= 31;
 
     if (!io_buffer_size || size / io_buffer_size > maxblocks)