AP_HAL: Improve ByteBuffer::set_size

So it doesn't delete and re-create the buffer if the size
happens to be the same. Still resets the buffer content.

libraries/AP_HAL/utility/RingBuffer.cpp

@@ -19,16 +19,20 @@ ByteBuffer::~ByteBuffer(void)
 }
 
 /*
-  caller is responsible for locking in set_size()
+ * Caller is responsible for locking in set_size()
  */
 void ByteBuffer::set_size(uint32_t _size)
 {
-    uint8_t *oldbuf = buf;
+    uint8_t *oldbuf;
+
     head = tail = 0;
+    if (_size != size) {
         size = _size;
+        oldbuf = buf;
         buf = new uint8_t[size];
         delete[] oldbuf;
     }
+}
 
 uint32_t ByteBuffer::available(void) const
 {