request: add missing early return to complete error check

While $self->error will immediately send out a 4xx or 5xx response
anyhow its still good to cover against possible side effects (e.g.,
from future code in that branch) on the server and return directly.

Note that this is mostly for completeness sake, we already have
another check that covers this one for relevant cases in commit
580d540ea9.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/PVE/APIServer/AnyEvent.pm

@@ -1578,6 +1578,7 @@ sub push_request_header {
 			# if an '@' comes before the first slash proxy forwarding might consider
 			# the frist part of the url to be part of an authority...
 			$self->error($reqstate, 400, "invalid url");
+			return;
 		    }
 
 		    $self->{request_count}++; # only count valid request headers