If VRT_error is called with reason==NULL, WSL() will eventually call
strlen(0), which will cause a SIGSEGV on (Open)Solaris.  Fix this by
checking if the reason is empty and just use (null) in that case.

Fixes: 458



git-svn-id: http://www.varnish-cache.org/svn/branches/2.0@3970 d4fa192b-c00b-0410-8231-f00ffab90ce4