pnm: Fixed segfault in pnmenc

The pnmenc was not mapping the input buffers as video buffers. Because
of this, the video frame stride was not being set based on frame but
based on the caps, which make the assumption that the strides are a
power of 4. For input that is not a power of 4, this would lead to a
SIGSEGV.

https://bugzilla.gnome.org/show_bug.cgi?id=793419
1 file changed