Check some error returns from *asprintf()

Check some error returns from *asprintf()

This avoids these compiler warnings on Ubuntu 18.04
gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)

expand_path.c: In function ‘expand_token’:
expand_path.c:493:17: warning: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Wunused-result]
                 asprintf(&arg, "%.*s", (int)(token_end - colon - 1), colon + 1);
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
log.c: In function ‘fmtkv’:
log.c:646:5: warning: ignoring return value of ‘vasprintf’, declared with attribute warn_unused_result [-Wunused-result]
     vasprintf(&buf1, fmt, ap);
     ^~~~~~~~~~~~~~~~~~~~~~~~~

mech/context.c: In function ‘gss_mg_set_error_string’:
mech/context.c:212:5: warning: ignoring return value of ‘vasprintf’, declared with attribute warn_unused_result [-Wunused-result]
     (void) vasprintf(&str, fmt, ap);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mech/context.c: In function ‘_gss_mg_log_name’:
mech/context.c:319:6: warning: ignoring return value of ‘vasprintf’, declared with attribute warn_unused_result [-Wunused-result]
      (void) vasprintf(&str, fmt, ap);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mech/context.c: In function ‘_gss_mg_log_cred’:
mech/context.c:346:5: warning: ignoring return value of ‘vasprintf’, declared with attribute warn_unused_result [-Wunused-result]
     (void) vasprintf(&str, fmt, ap);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

kerberos5.c: In function ‘_kdc_set_e_text’:
kerberos5.c:338:5: warning: ignoring return value of ‘vasprintf’, declared with attribute warn_unused_result [-Wunused-result]
     vasprintf(&e_text, fmt, ap);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Andrew Bartlett <abartlet@samba.org>