avformat/mpeg: Don't free unintialized pointer

In order to fix a potential memleak upon failure, 0b8956b2 made sure that
a buffer given by a pointer was freed upon error. But this pointer was
only initialized upon use and in several cases (Clang gives no fewer
than 13 -Wsometimes-uninitialized warnings) this meant that an
uninitialized pointer was used to free a buffer. So initialize the
pointer.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/mpeg.c

@@ -720,7 +720,7 @@ static int vobsub_read_header(AVFormatContext *s)
     int i, ret = 0, header_parsed = 0, langidx = 0;
     MpegDemuxContext *vobsub = s->priv_data;
     size_t fname_len;
-    char *header_str;
+    char *header_str = NULL;
     AVBPrint header;
     int64_t delay = 0;
     AVStream *st = NULL;