LWIP::socket_close() can attempt to free a null pbuf structure

[nat45928]

Description of defect

When closing a TCP socket with LwIP debugging enabled I can see a debug statement pbuf_free(p == NULL) was called. It appears change 3b178a0 modified a statement from netbuf_delete() to pbuf_free(). The netbuf_delete() function includes a null check which is not included in pbuf_free().

Target(s) affected by this defect ?

NUCLEO_F767ZI

Toolchain(s) (name and version) displaying this defect ?

GCC_ARM 9-2019q4

What version of Mbed-os are you using (tag or sha) ?

de798c4

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

mbed-cli 1.10.2

How is this defect reproduced ?

n/a

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOTRIAGE-2490

[0xc0170]

@kjbracey-arm Please review

[kjbracey]

I agree, it can do this on socket_close. Looks fine in the other part of the change in socket_recv. I'll make a patch.