eval: fix 'warning: ignoring return value of strtod, declared with attribute warn_unused_result'

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
r3d: fix division by 0 with 0 sample rate

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>