ffserver: local OOB write with custom program name

When the command line for children is created, it is assumed that
my_program_name always ends with "ffserver", which doesn't have to
be true if ffserver is called through a symbolic link.

In such a case, it could be that not enough space for "ffmpeg" is
available at the end, leading to a buffer overflow.

One example would be:

$ ln -s /usr/bin/ffserver ~/f; ~/f

As this is only a local buffer overflow, i.e. is based on a weird
program call, this has NO security impact.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>