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 <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/17690)