configure: Disable -Wmaybe-uninitialized by default

It is by definition unreliable and causes pointless noise on valid
code.
h263: Return meaningful errors
h263: Check init_get_bits return value

And use init_get_bits8 to check for integer overflows while at it.

CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
avio: K&R formatting cosmetics