formpost: avoid silent snprintf() truncation

formpost: avoid silent snprintf() truncation

The previous use of snprintf() could make libcurl silently truncate some
input data and not report that back on overly large input, which could
make data get sent over the network in a bad format.

Example:

 $ curl --form 'a=b' -H "Content-Type: $(perl -e 'print "A"x4100')"