BUG/MINOR: sample: Make the field converter compatible with -m found

Previously an expression like:

    path,field(2,/) -m found

always returned `true`.

Bug exists since the `field` converter exists. That is:
f399b0debf

The fix should be backported to 1.6+.

reg-tests/converter/field.vtc

@@ -0,0 +1,39 @@
+varnishtest "field converter Test"
+
+feature ignore_unknown_macro
+
+server s1 {
+	rxreq
+	txresp
+} -repeat 3 -start
+
+haproxy h1 -conf {
+    defaults
+	mode http
+	timeout connect 1s
+	timeout client  1s
+	timeout server  1s
+
+    frontend fe
+	bind "fd@${fe}"
+
+	#### requests
+	http-request set-var(txn.uri) path
+	http-response set-header Found %[var(txn.uri),field(3,/)] if { var(txn.uri),field(3,/) -m found }
+
+	default_backend be
+
+    backend be
+	server s1 ${s1_addr}:${s1_port}
+} -start
+
+client c1 -connect ${h1_fe_sock} {
+	txreq -url "/foo/bar/baz"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "bar"
+	txreq -url "/foo"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "<undef>"
+} -run

src/sample.c

@@ -2137,7 +2137,7 @@ static int sample_conv_field(const struct arg *arg_p, struct sample *smp, void *
 	/* Field not found */
 	if (field != arg_p[0].data.sint) {
 		smp->data.u.str.data = 0;
-		return 1;
+		return 0;
 	}
 found:
 	smp->data.u.str.data = end - start;