[6.0] f493f2b10 Remember to zero h2->mailcall on h2 rxbody vfp failure

[Dridi Boukelmoune]

commit f493f2b107e5e3f7f25ca0684b7cbc98c8d2d35b
Author: Dag Haavi Finstad <daghf at varnish-software.com>
Date:   Tue Jul 3 16:05:00 2018 +0200

    Remember to zero h2->mailcall on h2 rxbody vfp failure
    
    Fixes: #2572

diff --git a/bin/varnishd/http2/cache_http2_proto.c b/bin/varnishd/http2/cache_http2_proto.c
index f4cc294e5..3851089cd 100644
--- a/bin/varnishd/http2/cache_http2_proto.c
+++ b/bin/varnishd/http2/cache_http2_proto.c
@@ -835,8 +835,10 @@ h2_vfp_body_fini(struct vfp_ctx *vc, struct vfp_entry *vfe)
 		    H2SE_REFUSED_STREAM);
 		Lck_Lock(&h2->sess->mtx);
 		r2->error = H2SE_REFUSED_STREAM;
-		if (h2->mailcall == r2)
+		if (h2->mailcall == r2) {
+			h2->mailcall = NULL;
 			AZ(pthread_cond_signal(h2->cond));
+		}
 		Lck_Unlock(&h2->sess->mtx);
 	}
 }