rand: Add missing check for rand_get_global
rand: Add missing check for rand_get_global

As the potential failure of the rand_get_global(),
for example fail to get lock, 'dgbl' could be NULL
pointer and be dereferenced later.
Therefore, it should be better to check it and return
error if fails, like RAND_get0_primary() and other callers.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>

Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
Reviewed-by: Shane Lontis <452ce16516ceed26291fe7de3f8b53540d83864e@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/17690)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl