HAL_Linux: fixed some valgrind errors

libraries/AP_HAL_Linux/I2CDriver.cpp

@@ -162,6 +162,9 @@ uint8_t LinuxI2CDriver::readRegisters(uint8_t addr, uint8_t reg,
     nmsgs : 2
     };
 
+    // prevent valgrind error
+    memset(data, 0, len);
+
     if (ioctl(_fd, I2C_RDWR, &i2c_data) == -1) {
         return 1;
     }
@@ -207,6 +210,7 @@ uint8_t LinuxI2CDriver::readRegister(uint8_t addr, uint8_t reg, uint8_t* data)
         return 1;
     }
     union i2c_smbus_data v;
+    memset(&v, 0, sizeof(v));
     if (_i2c_smbus_access(_fd,I2C_SMBUS_READ, reg,
                           I2C_SMBUS_BYTE_DATA, &v)) {
         return 1;

libraries/AP_HAL_Linux/SPIDriver.cpp

@@ -109,6 +109,11 @@ void LinuxSPIDeviceDriver::transaction(const uint8_t *tx, uint8_t *rx, uint16_t
     spi[0].speed_hz      = _speed;
     spi[0].bits_per_word = _bitsPerWord;
     spi[0].cs_change     = 0;
+
+    if (rx != NULL) {
+        // keep valgrind happy
+        memset(rx, 0, len);
+    }
  
     ioctl(_fd, SPI_IOC_MESSAGE(1), &spi);
     cs_release();

libraries/AP_HAL_Linux/UARTDriver.cpp

@@ -163,6 +163,7 @@ void LinuxUARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
     if (b != 0 && _rd_fd == _wr_fd) {
         // set the baud rate
         struct termios t;
+        memset(&t, 0, sizeof(t));
         tcgetattr(_rd_fd, &t);
         cfsetspeed(&t, b);
         // disable LF -> CR/LF