avutil/Makefile: add softfloat to TESTPROGS

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/softfloat: Fix dependence on signed overflow in av_normalize1_sf()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avutil/softfloat: add some asserts

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>