Check if num is 0 before trying to malloc memory. Otherwise for client hellos...

Check if num is 0 before trying to malloc memory. Otherwise for client hellos without extensions SSL_client_hello_get1_extensions_present will return MALLOC_FAILURE.

Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8756)