libpam_misc: fix an inconsistency in handling memory allocation errors

libpam_misc: fix an inconsistency in handling memory allocation errors

When misc_conv fails to allocate memory for pam_response array, it
returns PAM_CONV_ERR.  However, when read_string fails to allocate
memory for a response string, it loses the response string and silently
ignores the error, with net result as if EOF has been read.

* libpam_misc/misc_conv.c (read_string): Use strdup instead of x_strdup,
the latter is of no benefit in this case.
Do not ignore potential memory allocation errors returned by strdup,
forward them to misc_conv.