configfile: fix EOF handling

configfile: fix EOF handling

Currently we can end up passing EOF to isspace(), which is in fact
libgit's sane_isspace which does:

	((sane_ctype[(unsigned char)(x)] & (GIT_SPACE)) != 0)

It is very unlikely that EOF cast to "unsigned char" will end up in a
character that has the GIT_SPACE bit set, but the standard only requires
that EOF be a negative integer, so it could access any value in the
sane_ctype array.

If it does end up returning true for isspace() then this loop will never
terminate, so handle EOF as a special value in the same way as the other
loops in this function.

Signed-off-by: John Keeping <john@keeping.me.uk>