Fix stm32 eeprom handling (#205)

* Update stm32_platform.cpp

Addresses a side effect of https://github.com/thelsing/knx/pull/177, which causes the eeprom buffer to be overwritten every time getEepromBuffer is called. This prevented programming of STM32 boards as their initial bus address was lost before programming.

* Code style fix

src/stm32_platform.cpp

@@ -31,17 +31,22 @@ void Stm32Platform::restart()
 }
 
 uint8_t * Stm32Platform::getEepromBuffer(uint16_t size)
+{
+    // check if the buffer already exists
+    if (_eepromPtr == nullptr) // we need to initialize the buffer first
     {
         if (size > E2END + 1)
         {
             fatalError();
         }
+
         _eepromSize = size;
-    delete [] _eepromPtr;
         _eepromPtr = new uint8_t[size];
         eeprom_buffer_fill();
         for (uint16_t i = 0; i < size; ++i)
             _eepromPtr[i] = eeprom_buffered_read_byte(i);   
+    }
+    
     return _eepromPtr;
 }