Fixes parsing small timebases from expressions (where the expression API
converts the result to double), like in this command line:

ffprobe -f lavfi -i testsrc=d=1,settb=1/2000000000 -show_streams -show_entries stream=time_base

Before the patch timebase was parsed as 1/1999999999.
Signed-off-by: Marton Balint <cus@passwd.hu>