avformat/mpeg: Don't free unintialized pointer

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>