Bug 1296152: [ffmpeg] Don't use all CPUs available when decoding. r=mattwoodrow

MozReview-Commit-ID: C4CTiC9H79A

--HG--
extra : rebase_source : 05c37856dc72c0b08e23c523cfa41e011b3802ea

dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp

@@ -145,7 +145,8 @@ FFmpegVideoDecoder<LIBAV_VER>::InitCodecContext()
     decode_threads = 2;
   }
 
-  decode_threads = std::min(decode_threads, PR_GetNumberOfProcessors());
+  decode_threads = std::min(decode_threads, PR_GetNumberOfProcessors() - 1);
+  decode_threads = std::max(decode_threads, 1);
   mCodecContext->thread_count = decode_threads;
   if (decode_threads > 1) {
     mCodecContext->thread_type = FF_THREAD_SLICE | FF_THREAD_FRAME;