AP_Scripting: fixed a memory leak in socket recv()

we need to allocate the memory in the lua heap, otherwise a failure
leaks global memory

libraries/AP_Scripting/lua_bindings.cpp

@@ -1003,21 +1003,23 @@ int SocketAPM_recv(lua_State *L) {
     SocketAPM * ud = *check_SocketAPM(L, 1);
 
     const uint16_t count = get_uint16_t(L, 2);
-    uint8_t *data = (uint8_t*)malloc(count);
-    if (data == nullptr) {
-        return 0;
-    }
 
+    // create a buffer sized to hold the number of bytes the user
+    // wants to read. This will fault if the memory is not available
+    luaL_Buffer b;
+    uint8_t *data = (uint8_t *)luaL_buffinitsize(L, &b, count);
+
+    // read up to that number of bytes
     const auto ret = ud->recv(data, count, 0);
     if (ret < 0) {
-        free(data);
-        return 0;
+        return 0; // error, return nil
     }
 
     int retcount = 1;
 
-    // push data to lua string
-    lua_pushlstring(L, (const char *)data, ret);
+    // push the buffer as a string, truncated to the number of bytes
+    // actually read
+    luaL_pushresultsize(&b, ret);
 
     // also push the address and port if available
     uint32_t ip_addr;
@@ -1028,8 +1030,6 @@ int SocketAPM_recv(lua_State *L) {
         retcount += 2;
     }
 
-    free(data);
-
     return retcount;
 }