ext/etc/etc.c (etc_systmpdir): set default tmplen correctly

Fixup r47826.

This fixes the following test failure for me:
TestRequire#test_tainted_loadpath [test/ruby/test_require.rb:331]:
SecurityError expected but nothing was raised.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47841 b2dd03c8-39d4-4d8f-98ff-823fe69b080e