This patch introduces mem_get_str0() and mem_as_str0(). Function
mem_get_str0() is used to receive NULL-terminated string from MEM. If
value of MEM is not NULL-terminated string, it is converted to
NULL-terminated string if possible. MEM is not changed. Function
mem_as_str0() is also used to receive NULL-terminated string from MEM,
however if MEM does not contain NULL-terminated string it converts MEM
to MEM that contains NULL-terminated string and returns its value.

Part of #5818