qt: Fix an rfc2253 parser flaw

* lang/qt/src/dn.cpp (parse_dn_part): Fix parser.

This could in theory result in reading bytes after a after Nul in a
string and thus possible segv on unallocated memory or reading other
parts of the memory. However, it is harmless because the rfc2253
strings have been received from GnuPG which is expected to emit
correct syntax.

• GnuPG-bug-id: T5037