Fix -O3 -Werror=unused-result build in dcache.c (#420)

Fix -O3 -Werror=unused-result build in dcache.c (#420)

* Fix -O3 -Werror=unused-result build in dcache.c

gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.10)
with -O3 -Werror=unused-result

../lib/krb5/dcache.c:85:5: error: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Werror=unused-result]
     asprintf(&path, "%s/primary-XXXXXX", dc->dir);
     ^
../lib/krb5/dcache.c: In function ‘primary_create’:
../lib/krb5/dcache.c:56:5: error: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Werror=unused-result]
     asprintf(&primary, "%s/primary", dc->dir);
     ^
../lib/krb5/dcache.c: In function ‘dcc_gen_new’:
../lib/krb5/dcache.c:423:5: error: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Werror=unused-result]
     asprintf(&name, ":%s/tktXXXXXX", dc->dir);
     ^
../lib/krb5/dcache.c: In function ‘dcc_resolve’:
../lib/krb5/dcache.c:340:2: error: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Werror=unused-result]
  asprintf(&dc->name, ":%s/%s", dc->dir, residual);
  ^
../lib/krb5/dcache.c:348:5: error: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Werror=unused-result]
     asprintf(&filename, "FILE%s", dc->name);
     ^
cc1: all warnings being treated as errors

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
* Update dcache.c

When asprintf() fails it is not guaranteed that the output variable will be NULL on all platforms and releases.

* Update dcache.c