strftime: don't assume a byte count fits in 'int'

strftime: don't assume a byte count fits in 'int'

* lib/strftime.c (add): Don't assume first arg fits in 'int'.  I
found this problem by static analysis, using gcc -Wstrict-overflow
(GCC 4.5.2, x86-64).  This reported an optimization that depended
on an integer overflow having undefined behavior, but it turns out
that the argument is a size, which might not fit in 'int' anyway,

2011-03-20  Paul Eggert  <eggert@cs.ucla.edu>