windows: Avoid leak of write buffer on realloc failure.

windows: Avoid leak of write buffer on realloc failure.

VS2019 IntelliSense reported:

Warning	C6308: 'realloc' might return null pointer: assigning null
               pointer to 'port->write_buf', which is passed as an
	       argument to 'realloc', will cause the original memory
	       block to be leaked.

This is correct, we would leak the buffer on a realloc failure.

Put the realloc result in a separate variable and handle the error
path before assigning the result to port->write_buf.