Add a missing boundary check for Range requests

Fixes #1323
Spotted & fix by:	gquintard

bin/varnishd/cache/cache_http1_deliver.c

@@ -126,6 +126,8 @@ v1d_dorange(struct req *req, const char *r)
 		}
 		if (!has_low) {
 			low = req->obj->len - high;
+			if (low < 0)
+				low = 0;
 			high = req->obj->len - 1;
 		}
 	} else

bin/varnishtest/tests/c00034.vtc

@@ -84,4 +84,9 @@ client c1 {
 	rxresp
 	expect resp.status == 206
 	expect resp.bodylen == 100
+
+	txreq -hdr "Range: bytes=-101" 
+	rxresp 
+	expect resp.status == 206 
+	expect resp.bodylen == 100 
 } -run