bug#54155: -f switch fails with large files on 3.x branch

[Marco Marsala]

This command:

grep -F -f b.txt a.txt

returns just first occurrence with grep 3.10 (mingw64, 4GB RAM machine) 
if files are bigger (b.txt ~ 1 MB, a.txt ~ 200 MB), works as expected 
with smaller files.

grep 2.10 on Debian (same RAM amount) works as expected even with such 
bigger files.