Eliminate complier warning and remove superfluous call to empty()

Appears when complied with -O3 optimization level

Compile: UARTSerial.cpp
../drivers/UARTSerial.cpp: In member function 'void mbed::UARTSerial::tx_irq()':
../drivers/UARTSerial.cpp:314:31: warning: 'data' may be used uninitialized in this function [-Wmaybe-uninitialized]
         SerialBase::_base_putc(data);

Author: Paul Thompson

drivers/UARTSerial.cpp

@@ -305,12 +305,11 @@ void UARTSerial::rx_irq(void)
 void UARTSerial::tx_irq(void)
 {
     bool was_full = _txbuf.full();
+    char data;
 
     /* Write to the peripheral if there is something to write
      * and if the peripheral is available to write. */
-    while (!_txbuf.empty() && SerialBase::writeable()) {
-        char data;
-        _txbuf.pop(data);
+    while (SerialBase::writeable() && _txbuf.pop(data)) {
         SerialBase::_base_putc(data);
     }