http: Don't expect an overflowed workspace on failure

If we lose a header from http_SetHeader() we don't formally run out of
workspace prior to calling http_fail().

Refs #3410

bin/varnishd/cache/cache_http.c

@@ -117,7 +117,7 @@ http_fail(const struct http *hp)
 	VSC_C_main->losthdr++;
 	WS_Id(hp->ws, id);
 	VSLb(hp->vsl, SLT_Error, "out of workspace (%s)", id);
-	assert(WS_Overflowed(hp->ws));
+	WS_MarkOverflow(hp->ws);
 }
 
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