libx264: Don't leave max_b_frames as -1 if the user didn't set it

max_b_frames is initialized to -1 for libx264, to allow
distinguishing between an explicit user set 0 and a default not
touched 0 (see bb73cda2).

If max_b_frames is left as -1, this affects dts generation (where
expressions like max_b_frames != 0 are used), so make sure it is
left at the default 0 after the libx264 init function returns.

This avoids unnecessarily producing dts != pts when using
profile=baseline.

Signed-off-by: Martin Storsjö <martin@martin.st>
