fixed compiler warnings

USBDrive_Airplane_Data_Logger/USBDrive_Airplane_Data_Logger.ino

@@ -247,7 +247,7 @@ time_t getdatetime(){
   // gets time from GPS
   // returns time_t with nanoseconds as a float
   time_t t;
-  TimeElements tm;
+  //TimeElements tm;
 
   t = 0;
 
@@ -284,7 +284,7 @@ void loop(void) {
     if (i1 == Serial1RxBufferSIZE - 1) {
       MSG_OVFL = true;
       msg1 = i1;
-    } else GPSRx[i1]=NULL;
+    } else GPSRx[i1]='\0';
   }
   
   // process until end of message or overflow
@@ -296,7 +296,7 @@ void loop(void) {
     if (i2 == Serial2RxBufferSIZE - 1) {
       MSG_OVFL = true;
       msg2 = i2;
-    } else EFISRx[i2]=NULL;
+    } else EFISRx[i2]='\0';
   }
   
   // process until end of message or overflow
@@ -308,7 +308,7 @@ void loop(void) {
     if (i3 == Serial3RxBufferSIZE - 1) {
       MSG_OVFL = true;
       msg3 = i3;
-    } else EMSRx[i3]=NULL;
+    } else EMSRx[i3]='\0';
   }