Assertion fails within STM32 MAC driver.

I am using Ethernet on an STM32F427, and I am plagued with an assertion failing constantly.

It is on file `stm32_eth.c`, line 1372.:
`DEBUGASSERT(dev->d_len == 0 && dev->d_buf == NULL);`

Checking with my debugger I see that `dev->d_len` has a non zero value (which however seems sane, not garbage).
`dev->d_buf` is NULL, as it ought to be.

I had set `CONFIG_NET_ETH_PKTSIZE` to 640, and this problem appeared quite often.
Now, I tried to set this config to 1514 and to my surprise, the problem vanished. I haven't seen this assertion failing again.

Maybe the problem is solved, or maybe it is now so scarce that I haven't noticed it?

I am thinking of two cases here:
* Either there is something "special" with the value 1514, and the driver does not work with smaller values.
* Or having a small buffer triggers the execution of some buggy code (for example by dropping large packets).

I guess that if it is the second case, the problem is still there but it is just temporarily masked for me.

Start in stm32_eth.c at line 1372 and reproduce the assertion with CONFIG_NET_ETH_PKTSIZE set to 640, comparing behavior with 1514. Inspect the packet state values for d_len and d_buf, and verify that the driver handles the smaller buffer size without triggering the assertion.

Written by the indexing model from the issue text.