Fix LWIP warning issues found by Coverity scan

[tymoteuszblochmobica]

Description

During Coverity scan several potential defects was found.
This PR fix them.

lwip_dns.c in function: dns_add_interface_server
CID 1399051 (#1 of 1): Buffer not null terminated (BUFFER_SIZE_WARNING)10. buffer_size_warning: Calling strncpy with a maximum size argument of 6 bytes on destination array new_interface_server->interface_name of size 6 bytes might leave the destination string unterminated.
line 434 strncpy(new_interface_server->interface_name, interface_name, INTERFACE_NAME_MAX_SIZE);

lwip_ip4_frag.c in function: ip_reass_free_complete_datagram

CID 1373164 (#1 of 1): Dereference after null check (FORWARD_NULL)7. var_deref_model: Passing null pointer prev to ip_reass_dequeue_datagram, which dereferences it. [show details]
line 209 ip_reass_dequeue_datagram(ipr, prev);

lwip_ip4_frag.c in function: ip_reass

CID 1373163 (#1-2 of 2): Dereference after null check (FORWARD_NULL)38. var_deref_model: Passing null pointer ipr_prev to ip_reass_dequeue_datagram, which dereferences it. [show details]
line 663 ip_reass_dequeue_datagram(ipr, ipr_prev);

lwip_api_msg.c in function: lwip_netconn_do_connected

CID 1373162 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)10. var_deref_model: Passing null pointer op_completed_sem to sys_sem_signal, which dereferences it. [show details]
line 1336 sys_sem_signal(op_completed_sem);

https://scan.coverity.com/projects/mbed-os-d90561a2-1b71-4656-a2f4-1721081c2f84?tab=overview

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

@SeppoTakalo
@mikaleppanen
@kjbracey-arm

Release Notes

[ciarmcom]

@tymoteuszblochmobica, thank you for your changes.
@SeppoTakalo @kjbracey-arm @mikaleppanen @ARMmbed/mbed-os-ipcore @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

@tymoteuszblochmobica For future reference, include the warnings being fixed in the commit msg (not only on Github)

I added them to the merge commit msg