file.c: clear tmp buffer on failures

Reduces GC malloc pressure (MAXPATHLEN is 4096 on my system),
rb_find_file_safe hits this path at least twice every time
ruby starts.

* file.c (rb_find_file_ext_safe): clear tmp buffer on failure
  (rb_find_file_safe): ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47664 b2dd03c8-39d4-4d8f-98ff-823fe69b080e