avformat/rmdec: Actually return value < 0 on read_header failure

avformat/rmdec: Actually return value < 0 on read_header failure

The RealMedia demuxer's read_header function initially initializes ret,
the variable designated for the return variable to -1. Afterwards, chunks
of the file are parsed in a loop until an error happens or until the actual
frame data is encountered. If the first function whose return
value is put into ret doesn't fail, then ret contains a value >= 0
(actually == 0) and this is what will be returned if an error is
encountered afterwards.

This is a regression since 35bbc1955a58ba74552c50d9161084644f00bbd3.
Before that, ret had never been overwritten with a nonnegative value.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>