Give _int_pin and _reset_pin sane default values

Assign sane default values to _int_pin and _reset_pin.  Otherwise they
get assigned zero and all the "if (_int_pin == -1)" checks fail and
bad things happen.

Author: Bob Ham

src/SparkFun_BNO08x_Arduino_Library.cpp

@@ -42,7 +42,7 @@
 
 #include "SparkFun_BNO08x_Arduino_Library.h"
 
-int8_t _int_pin, _reset_pin;
+int8_t _int_pin = -1, _reset_pin = -1;
 static TwoWire *_i2cPort = NULL;		//The generic connection to user's chosen I2C hardware
 static SPIClass *_spiPort = NULL;  		//The generic connection to user's chosen SPI hardware
 static uint8_t _deviceAddress = BNO08x_DEFAULT_ADDRESS; //Keeps track of I2C address. setI2CAddress changes this.
@@ -1653,4 +1653,4 @@ static bool spi_write(const uint8_t *buffer, size_t len,
   _spiPort->endTransaction();
 
   return true;
-}
+}