avcodec/libx264: silence -Waddress

This patch moves the pointer validity check outside the macro,
and silences the -Waddress observed with GCC 5.2.

Note that this changes the error message slightly, from:
"bad option..." to "Error parsing option...".

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
