random: Initialize the mode field when seeding in php_random_default_status() (php#13608)

This is not just an issue due to missing initialization since moving the state
struct directly into the module globals. In earlier versions changing the mode
to `MT_RAND_PHP` within a single request would also affect the mode for
subsequent requests.

Original commit message follows:

This is a follow-up fix for phpGH-13579. The issue was detected in the nightly
MSAN build.

(cherry picked from commit bf0abd1)

Author: TimWolla

ext/random/random.c

@@ -334,6 +334,7 @@ PHPAPI php_random_status *php_random_default_status(void)
 	php_random_status *status = RANDOM_G(mt19937);
 
 	if (!RANDOM_G(mt19937_seeded)) {
+		((php_random_status_state_mt19937 *)status->state)->mode = MT_RAND_MT19937;
 		php_random_mt19937_seed_default(status->state);
 		RANDOM_G(mt19937_seeded) = true;
 	}