Properly handle PKCS11 label in PKINIT

The CK_TOKEN_INFO label field is defined to be zero-filled, but it may
not be zero-terminated if all bytes of the field are used.  Use only
length-counted operations to process it.  Also avoid underrunning the
buffer pointer if the label is empty or contains only whitespace.

ticket: 7916
target_version: 1.12.2
tags: pullup