common: APN with a dot in second position are not invalid

APN with a dot in second position (example: "t.est") are wrongly
considered invalid.

src/common.c

@@ -709,7 +709,7 @@ const char *registration_tech_to_string(int tech)
 gboolean is_valid_apn(const char *apn)
 {
 	int i;
-	int last_period = 0;
+	int last_period = -1;
 
 	if (apn == NULL)
 		return FALSE;