vsnprintf.c: fix string precision

* vsnprintf.c (BSD_vfprintf): fix string width when precision is
  given.  as the result of `memchr` is NULL or its offset from the
  start cannot exceed the size, the comparison was always false.
  [ruby-core:62737] [Bug #9861]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46098 b2dd03c8-39d4-4d8f-98ff-823fe69b080e