configure: Add usan to the toolchain presets

clang-3.4 and gcc-4.9 have it.
avconv: Do not divide by zero
h264: Lower bound check for slice offsets

And use the value from the specification.

Sample-Id: 00000451-google
Found-by: Mateusz j00ru Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>