BUG/MEDIUM: da: stop DeviceAtlas processing in the convertor if there is no input.

In case a HTTP header modifier, like req*del, is used, the User-Agent would be removed
and cause a segfault, hence the work is stopped in due time.
diff --git a/src/da.c b/src/da.c
index e1886b8..a214b31 100644
--- a/src/da.c
+++ b/src/da.c
@@ -232,7 +232,7 @@
 	char useragentbuf[1024] = { 0 };
 	int i;
 
-	if (global.deviceatlas.daset == 0) {
+	if (global.deviceatlas.daset == 0 || smp->data.u.str.len == 0) {
 		return 1;
 	}