Fix maybe uninitialized warnings in random.c

Fixes the following compiler warnings:

    random.c: In function `random_init`:
    random.c:416:38: warning: `rng` may be used uninitialized in this function [-Wmaybe-uninitialized]
      416 |     unsigned int major = rng->version.major;
          |                          ~~~~~~~~~~~~^~~~~~
    random.c: In function `random_bytes`:
    random.c:1284:8: warning: `rng` may be used uninitialized in this function [-Wmaybe-uninitialized]
    1284 |     rng->get_bytes(rnd, ptr, n);
         |     ~~~^~~~~~~~~~~
    random.c:1299:34: note: `rng` was declared here
    1299 |     const rb_random_interface_t *rng;
         |                                  ^~~
    random.c: In function `rand_random_number`:
    random.c:1606:12: warning: `rng` may be used uninitialized in this function [-Wmaybe-uninitialized]
    1606 |     return rand_range(obj, rng, rnd, vmax);
         |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    random.c:1624:34: note: `rng` was declared here
    1624 |     const rb_random_interface_t *rng;
          |                                  ^~~
    random.c: In function `random_rand`:
    random.c:1120:15: warning: `rng` may be used uninitialized in this function [-Wmaybe-uninitialized]
    1120 |     return rng->get_int32(rnd);
         |            ~~~^~~~~~~~~~~
    random.c:1573:34: note: `rng` was declared here
    1573 |     const rb_random_interface_t *rng;
         |                                  ^~~

Author: peterzhu2118

random.c

@@ -405,7 +405,7 @@ rand_init(const rb_random_interface_t *rng, rb_random_t *rnd, VALUE seed)
 static VALUE
 random_init(int argc, VALUE *argv, VALUE obj)
 {
-    const rb_random_interface_t *rng;
+    const rb_random_interface_t *rng = NULL;
     rb_random_t *rnd = try_get_rnd(obj, &rng);
 
     if (!rng) {
@@ -1296,7 +1296,7 @@ rand_bytes(const rb_random_interface_t *rng, rb_random_t *rnd, long n)
 static VALUE
 random_bytes(VALUE obj, VALUE len)
 {
-    const rb_random_interface_t *rng;
+    const rb_random_interface_t *rng = NULL;
     rb_random_t *rnd = try_get_rnd(obj, &rng);
     return rand_bytes(rng, rnd, NUM2LONG(rb_to_int(len)));
 }
@@ -1570,7 +1570,7 @@ static VALUE rand_random(int argc, VALUE *argv, VALUE obj, const rb_random_inter
 static VALUE
 random_rand(int argc, VALUE *argv, VALUE obj)
 {
-    const rb_random_interface_t *rng;
+    const rb_random_interface_t *rng = NULL;
     rb_random_t *rnd = try_get_rnd(obj, &rng);
     VALUE v = rand_random(argc, argv, obj, rng, rnd);
     check_random_number(v, argv);
@@ -1621,7 +1621,7 @@ rand_random(int argc, VALUE *argv, VALUE obj, const rb_random_interface_t *rng,
 static VALUE
 rand_random_number(int argc, VALUE *argv, VALUE obj)
 {
-    const rb_random_interface_t *rng;
+    const rb_random_interface_t *rng = NULL;
     rb_random_t *rnd = try_get_rnd(obj, &rng);
     VALUE v = rand_random(argc, argv, obj, rng, rnd);
     if (NIL_P(v)) v = rand_random(0, 0, obj, rng, rnd);