[v3] Fix out-of-bounds search in num_get::_M_extract_int

[Paolo Carlini]

Richard Sandiford wrote:

>Patch tested on mips64{,el}-linux-gnu.  It brings us back to zero
>FAILs in libstdc++-v3.  OK for 3.4 and HEAD?
>  
>
Yes, and *many* thanks! Hopefully the last good catch of yours in this 
area... ;)

Paolo.