yuv4mpegdec: initialize field_order in yuv4_read_header()

CC: libav-stable@libav.org
Bug-Id: CID 1265721

libavformat/yuv4mpegdec.c

@@ -38,8 +38,8 @@ static int yuv4_read_header(AVFormatContext *s)
         rated =  0, aspectn =  0, aspectd = 0;
     enum AVPixelFormat pix_fmt = AV_PIX_FMT_NONE, alt_pix_fmt = AV_PIX_FMT_NONE;
     enum AVChromaLocation chroma_sample_location = AVCHROMA_LOC_UNSPECIFIED;
+    enum AVFieldOrder field_order = AV_FIELD_UNKNOWN;
     AVStream *st;
-    enum AVFieldOrder field_order;
 
     for (i = 0; i < MAX_YUV4_HEADER; i++) {
         header[i] = avio_r8(pb);