LA-3887 Fix STC deregister
LA-3887 Fix STC deregister

ca_stc_deregister would sometimes provide a null-terminated c string
by chance in AutoCArray. Change it to always create a null-terminated
c string by using create_string_buffer directly. No len arg will
result in a c string with null-termination.

Change-Id: Idc29e555a2f7064fd49e30d03e2747cef91263d4

gemalto/pycryptoki