gnrc_tftp: use of uninitialized values

[nmeum]

Description

The gnrc_tftp_server doesn't initialize all members of the tftp_context_t in gnrc_tftp_server causing an access of ununinitialised values when receiving messages in an unexpected order.

Steps to reproduce the issue

1. Build examples/gnrc_tftp with make all-valgrind
2. Run the application using make term-valgrind
3. Start the TFTP server using tftps start and wait until it started
4. Send a DATA tftp packet to the server, for example:

echo AAMAAGZvb2Jhcg== | base64 -d | nc -u '[ip-address%tap0]' 69

Expected results

Valgrind shouldn't report any accesses of uninitialised values.

Actual results

Valgrind reports various conditional jumps or moves which depend on uninitialised values.

[miri64]

Confirmed and fixed in #11773.