wpa_supplicant: Fix parsing errors on additional config file

wpa_supplicant: Fix parsing errors on additional config file

If the -I<config> argument is used and the referenced configuration file
cannot be parsed, wpa_config_read() ended up freeing the main
configuration data structure and that resulted in use of freed memory in
such an error case. Fix this by not freeing the main config data and
handling the error case in the caller.

Signed-off-by: Jouni Malinen <j@w1.fi>