Do not increment reqstate when headers are incomplete.

If the server got incomplete headers (I was able to reliably trigger
this with connection reuse), the reqstate variable was still incremented.

This resulted in the request state becoming inconsistent - notably,
HEAD requests believed they always included a checksum.  There are
likely other strange XrdHttp behaviors that could have been triggered
due to the same bug.

(cherry picked from commit 88dad6a)

src/XrdHttp/XrdHttpProtocol.cc

@@ -750,6 +750,7 @@ int XrdHttpProtocol::Process(XrdLink *lp) // We ignore the argument here
 
     if (!CurrentReq.headerok) {
       TRACEI(REQ, " rc:" << rc << "Header not yet complete.");
+      CurrentReq.reqstate--;
       // Waiting for more data
       return 1;
     }