input: decoder: fix data race on decoder format

We cannot read or write the decoder object without being under the fifo
lock, so ensure we locked first. It fixes some thread sanitizer issues
between decoder_Init() and vlc_input_decoder_Flush().

    WARNING: ThreadSanitizer: data race (pid=876901)
      Read of size 8 at 0x7b7000010028 by thread T25:
        #0 vlc_input_decoder_Flush ../../src/input/decoder.c:2317 (libvlccore.so.9+0x69d7a) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        #1 EsOutDestroyDecoder ../../src/input/es_out.c:2360 (libvlccore.so.9+0x716cf) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#2 EsOutUnselectEs ../../src/input/es_out.c:2534 (libvlccore.so.9+0x7a1bd) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#3 EsOutVaPrivControlLocked ../../src/input/es_out.c:3733 (libvlccore.so.9+0x75832) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#4 EsOutPrivControl ../../src/input/es_out.c:4043 (libvlccore.so.9+0x783ad) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        #5 es_out_in_vaPrivControl ../../src/input/es_out_timeshift.c:464 (libvlccore.so.9+0x81a06) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        #6 es_out_in_PrivControl ../../src/input/es_out_timeshift.c:474 (libvlccore.so.9+0x81a06)
        videolan#7 CmdExecutePrivControl ../../src/input/es_out_timeshift.c:1868 (libvlccore.so.9+0x81b78) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#8 PrivControlLocked ../../src/input/es_out_timeshift.c:758 (libvlccore.so.9+0x840be) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#9 PrivControl ../../src/input/es_out_timeshift.c:817 (libvlccore.so.9+0x840be)
        videolan#10 es_out_vaPrivControl ../../src/input/es_out.h:105 (libvlccore.so.9+0x86f7c) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#11 es_out_PrivControl ../../src/input/es_out.h:112 (libvlccore.so.9+0x86f7c)
        videolan#12 es_out_SetMode ../../src/input/es_out.h:119 (libvlccore.so.9+0x91ba9) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#13 End ../../src/input/input.c:1425 (libvlccore.so.9+0x91ba9)
        videolan#14 Run ../../src/input/input.c:431 (libvlccore.so.9+0x920d4) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)

      Previous write of size 8 at 0x7b7000010028 by thread T27:
        #0 decoder_Init ../../src/input/decoder_helpers.c:50 (libvlccore.so.9+0x6c1ce) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        #1 LoadDecoder ../../src/input/decoder.c:370 (libvlccore.so.9+0x672ae) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#2 DecoderThread_Reload ../../src/input/decoder.c:431 (libvlccore.so.9+0x67593) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#3 DecoderThread_ProcessInput ../../src/input/decoder.c:1595 (libvlccore.so.9+0x687f9) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        videolan#4 DecoderThread_DecodeBlock ../../src/input/decoder.c:1562 (libvlccore.so.9+0x6865b) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        #5 DecoderThread_ProcessInput ../../src/input/decoder.c:1667 (libvlccore.so.9+0x6895e) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)
        #6 DecoderThread ../../src/input/decoder.c:1795 (libvlccore.so.9+0x6a487) (BuildId: 1912c3713753e3bafbd8f9b735ec069a39be1f4d)

src/input/decoder.c

@@ -2313,9 +2313,8 @@ void vlc_input_decoder_Drain( vlc_input_decoder_t *p_owner )
 
 void vlc_input_decoder_Flush( vlc_input_decoder_t *p_owner )
 {
-    enum es_format_category_e cat = p_owner->dec.fmt_in->i_cat;
-
     vlc_fifo_Lock( p_owner->p_fifo );
+    enum es_format_category_e cat = p_owner->dec.fmt_in->i_cat;
 
     /* Empty the fifo */
     block_ChainRelease( vlc_fifo_DequeueAllUnlocked( p_owner->p_fifo ) );