samba-o3: fix -Werror=maybe-uninitialized in lib/mscat/mscat_pks7.c

samba-o3: fix -Werror=maybe-uninitialized in lib/mscat/mscat_pks7.c

samba-o3 test failed in ubuntu:1804 image with:

    ../../lib/mscat/mscat_pkcs7.c: In function ‘mscat_pkcs7_import_catfile’:
    ../../lib/mscat/mscat_pkcs7.c:143:18: error: ‘blob.length’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
      mscat_data.size = blob.length;
      ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
    ../../lib/mscat/mscat_pkcs7.c:142:18: error: ‘blob.data’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
      mscat_data.data = blob.data;
      ~~~~~~~~~~~~~~~~^~~~~~~~~~~
    ../../lib/mscat/mscat_pkcs7.c: In function ‘mscat_pkcs7_verify’:
    ../../lib/mscat/mscat_pkcs7.c:225:16: error: ‘blob.length’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
       ca_data.size = blob.length;
       ~~~~~~~~~~~~~^~~~~~~~~~~~~
    ../../lib/mscat/mscat_pkcs7.c:224:16: error: ‘blob.data’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
       ca_data.data = blob.data;
       ~~~~~~~~~~~~~^~~~~~~~~~~
    cc1: all warnings being treated as errors

Since in `mscat_read_file`, it may still return rc = 0 while goto error,
ends up with blob uninitialized.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>