Fix (harmless) memory leaks & replace magic number with constant

[eumpf0]

• Add preprocessor constant DEVLIST_ALLOC_INITIAL_LEN to replace magic number 64 in allocations of device info list. The named constant expresses meaning and makes bugs & memory-leaks due to missed code changes less likely, should one decide to change the number of slots to allocate in the device info list.

• Fix the same errorneous code in freeing the device info list at three points in the code:
  Calling fido_dev_info_free with length argument ndevs deallocates only the filled entries of the device info list, while leaving the empty entries dangling. This does not seem to leak any data, though.
  The commit fixes the code to actually deallocate the whole array.

[LDVG]

Add preprocessor constant DEVLIST_ALLOC_INITIAL_LEN ...

Would you oppose making this slightly shorter, e.g. DEVLIST_LEN?

Calling fido_dev_info_free with length argument ndevs deallocates only the filled entries of the device info list, while leaving the empty entries dangling. This does not seem to leak any data, though. The commit fixes the code to actually deallocate the whole array.

The current usage should not leak and is how libfido2 itself calls fido_dev_info_free() in its examples and tools. libfido2 does allow passing the entire length of the list even if only a subset of it was populated by fido_dev_info_manifest() so I'm not opposed to the change if we prefer that approach.

Would you mind a) splitting your changes into two commits (one for the constant, one for the changed fido_dev_info_free() calls); b) rewording the commit messages in accordance to my comments in your other PR; and c) reword the part about erroneous freeing/memory leaks?

Thanks!

[LDVG]

Adjusted according to previous comment.