bug#12350: Composites identified as primes in factor.c (when HAVE_GMP)

[Torbjorn Granlund]

I found a problem with the GMP integration.

We have a 100 byte buffer in the stdin reading code, which was adequate
before we used GMP, but now one might want to attempt to factor much
larger numbers.

We'll fix that, but not tonight.

-- 
Torbjörn