Fix wpa_config_parse_string() to null terminate printf decoded values

Fix wpa_config_parse_string() to null terminate printf decoded values

printf_decode() fills in a binary buffer and returns the length of
the written data. This did not use null termination since initial
use cases used the output as a binary value. However, Hotspot 2.0
cred block values are also using this for parsing strings. Those
cases could end up without proper null termination depending on what
os_malloc() ends up getting as the memory buffer. Fix these and make
printf_decode() more convenient by forcing the output buffer to be
null terminated.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>