Reduce a race condition... We doubly protect against a bogus vhostrec

pointer. The first test sets the pointer correctly no matter what
the value of vhostrec. When it's time to print out, we check again.
If the previous value was NULL, then no matter what, "NULL" will be
used for the hostname, even if the new vhostrec was changed between
the 2 tests to be valid. If the previous was OK, then the vhost value
will only be printed if it's still OK. There's still a window between
this final test and when the actual string is printed, since it's possible
(maybe) that right after the test, the vhostrec is no longer valid, but
assuming that the memory hasn't been released or overwritten, we still
point to a good string.
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@82441 13f79535-47bb-0310-9956-ffa450edef68