avutil/opt.c: fix bug when parsing option

fix these strange behavior:
-af flanger=delay=10 [OK]
-af flanger=delay=1e1 [OK]
-af flanger=delay=1e+1 [Error]
-af flanger=delay=0.1 [OK]
-af flanger=delay=1e-1 [Error]

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