Fix an off-by-one when copying the subject to the workspace

and add a test for it

Fixes #3

src/tests/b01.vtc

@@ -6,12 +6,19 @@ server s1 {
 } -start
 
 varnish v1 -vcl+backend {
+	import std;
 	import re from "${vmod_topbuild}/src/.libs/libvmod_re.so";
 
 	sub vcl_init {
 	        new foobar = re.regex("foobar");
 		new snafu = re.regex("snafu");
 		new bar = re.regex("bar");
+
+		if (bar.match("bar45678")) {
+			std.log(bar.backref(0, ""));
+		} else {
+			return (fail);
+		}
 	}
 
         sub vcl_recv {

src/vmod_re.c

@@ -174,7 +174,7 @@ match(VRT_CTX, vre_t *vre, VCL_STRING subject, struct vmod_priv *task)
 	}
 
 	task->len = sizeof(*ov);
-	len = strlen(subject);
+	len = strlen(subject) + 1;
 	if (WS_Inside(ctx->ws, subject, subject + len))
 		ov->subject = subject;
 	else if ((ov->subject = WS_Copy(ctx->ws, (const void *) subject, len))