avutil: av_x_if_null: remove const for the return value.

avutil: av_x_if_null: remove const for the return value.

This is the usual weirdness with const/non-const pointers through a function.
See the prototype of strchr for a standard example.

The function had to be moved down, below the point where stdint.h is
indirectly included.

Signed-off-by: Nicolas George <nicolas.george@normalesup.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>