MAM - Use of uninitialised value of size 8 (mam_api_destroy)

[kuhlmannmarkus]

Describe the bug
A Use of uninitialised value of size 8 appears.
Valgrind Info:
==10878== at 0x294A5D: mam_channel_t_set_remove_entry (mam_channel_t_set.c:37)
==10878== by 0x292F06: mam_channels_destroy (channel.c:81)
==10878== by 0x27F8A3: mam_api_destroy (api.c:306)

To Reproduce
Steps to reproduce the behavior:
Use mam_api_destroy in any way.
For example run mam/api/tests/test_api through valgrind. This serves as example.

Expected behavior
Use of uninitialised value of size 8 should not happen.

Desktop (please complete the following information):

• OS: Ubuntu 18.04