Cannot create empty map using cbor_new_definite_map() on some platforms.

[hglee]

Describe the bug

In cbor_new_definite_map(), _CBOR_DEPENDENT_NOTNULL is used to check whether .data has been allocated.

But on some platforms, likes NI LabWindows/CVI, cbor_alloc_multiple (i.e. malloc) returns NULL when requsted size is 0.

Therefore, cannot create empty map by _CBOR_DEPENDENT_NOTNULL check.

Return value of malloc(0) is implementation-defined, so it is not possible to create empty map on some platforms.

To Reproduce

Call cbor_new_definite_map(0) in NI LabWindows/CVI

Expected behavior

Create empty map using cbor_new_definite_map(0) even if malloc(0) returns NULL.

Environment

• libcbor 0.13.0
• NI LabWindows/CVI