Merge of r4965: upstream: fixed SIGSEGV with the "if" directive. Configuration like location / { set $true 1; if ($true) { proxy_pass http://backend; } if ($true) { # nothing } } resulted in segmentation fault due to NULL pointer dereference as the upstream configuration wasn't initialized in an implicit location created by the last if(), but the r->content_handler was set due to first if(). Instead of committing a suicide by dereferencing a NULL pointer, return 500 (Internal Server Error) in such cases, i.e. if uscf is NULL. Better fix would be to avoid such cases by fixing the "if" directive handling, but it's out of scope of this patch. Prodded by Piotr Sikora.