hw/net: Fix read of uninitialized memory in ftgmac100

hw/net: Fix read of uninitialized memory in ftgmac100

With the `size += 4` before the call to `crc32`, the CRC calculation
would overrun the buffer. Size is used in the while loop starting on
line 1009 to determine how much data to write back, with the last
four bytes coming from `crc_ptr`, so do need to increase it, but should
do this after the computation.

I'm unsure why this use of uninitialized memory in the CRC doesn't
result in CRC errors, but it seems clear to me that it should not be
included in the calculation.

Signed-off-by: Stephen Longfield <slongfield@google.com>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-Id: <20221220221437.3303721-1-slongfield@google.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>