BUG/MEDIUM: contrib/modsecurity: If host header is NULL, don't try to strdup it

I discovered this bug when running OWASP regression tests against HAProxy +
modsecurity-spoa (it's a POC to evaluate how it is working).  I found out that
modsecurity spoa will crash when the request doesn't have any Host header.

See the pull request #86 on github for details.

This patch must be backported to 1.9 and 1.8.
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