AP_ExternalAHRS_VectorNav: Move allocation to local scope

Move receive_buf to method scope allocation rather than class scope

libraries/SITL/SIM_VectorNav.cpp

@@ -213,6 +213,7 @@ void VectorNav::update(void)
         send_packet2();
     }
 
+    char receive_buf[50];
     const ssize_t n = read_from_autopilot(&receive_buf[0], ARRAY_SIZE(receive_buf));
     if (n > 0) {
         if (strncmp(receive_buf, "$VNRRG,01", 9) == 0) {

libraries/SITL/SIM_VectorNav.h

@@ -45,8 +45,6 @@ private:
     uint32_t last_pkt2_us;
     uint32_t last_type_us;
 
-    char receive_buf[50];
-
     void send_packet1();
     void send_packet2();
     void nmea_printf(const char *fmt, ...);