Tolerate a null address string in std.ip()

A regression from d7a81fe8 and the
followup changes from #2993. The new VSS_Resolve{One,First} functions
expect a non-null address string.

Conflicts:
	lib/libvmod_std/vmod_std_conversions.c

bin/varnishtest/tests/m00011.vtc

@@ -77,6 +77,11 @@ varnish v1 -vcl+backend {
 		set resp.http.port11 = std.port(debug.get_ip());
 		std.timestamp("1.2.3.4 80, p = 443");
 
+		debug.store_ip(std.ip(req.http.non-existent, server.ip));
+		set resp.http.ip12 = debug.get_ip();
+		set resp.http.port12 = std.port(debug.get_ip());
+		std.timestamp("NULL, server.ip");
+
 	}
 } -start
 
@@ -110,4 +115,6 @@ client c1 {
 	expect resp.http.port10 == ${s1_port}
 	expect resp.http.ip11 == 1.2.3.4
 	expect resp.http.port11 == 80
+	expect resp.http.ip12 == ${v1_addr}
+	expect resp.http.port12 == ${v1_port}
 } -run

lib/libvmod_std/vmod_std_conversions.c

@@ -81,10 +81,9 @@ VCL_IP
 vmod_ip(VRT_CTX, VCL_STRING s, VCL_IP d, VCL_BOOL n, VCL_STRING default_port)
 {
 	void *p;
-	VCL_IP retval;
+	VCL_IP retval = NULL;
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
-	AN(s);
 	AN(d);
 	assert(VSA_Sane(d));
 
@@ -94,8 +93,9 @@ vmod_ip(VRT_CTX, VCL_STRING s, VCL_IP d, VCL_BOOL n, VCL_STRING default_port)
 		return (NULL);
 	}
 
-	retval = VSS_ResolveFirst(p, s, default_port, AF_UNSPEC, SOCK_STREAM,
-	    n ? 0 : AI_NUMERICHOST|AI_NUMERICSERV);
+	if (s != NULL)
+		retval = VSS_ResolveFirst(p, s, default_port, AF_UNSPEC,
+		    SOCK_STREAM, n ? 0 : AI_NUMERICHOST|AI_NUMERICSERV);
 
 	if (retval != NULL)
 		return (retval);