Fix default value of global_init_slots

Not setting a value to global_init_slots causes get_envparam_size to
output a broken default value.

gc.c

@@ -11665,7 +11665,7 @@ ruby_gc_set_params(void)
     }
 
     /* RUBY_GC_HEAP_INIT_SLOTS */
-    size_t global_init_slots;
+    size_t global_init_slots = GC_HEAP_INIT_SLOTS;
     if (get_envparam_size("RUBY_GC_HEAP_INIT_SLOTS", &global_init_slots, 0)) {
         for (int i = 0; i < SIZE_POOL_COUNT; i++) {
             gc_params.size_pool_init_slots[i] = global_init_slots;

test/ruby/test_gc.rb

@@ -345,6 +345,8 @@ class TestGc < Test::Unit::TestCase
     assert_in_out_err([env, "-W0", "-e", "exit"], "", [], [], "[ruby-core:39795]")
     assert_in_out_err([env, "-W1", "-e", "exit"], "", [], [], "[ruby-core:39795]")
     assert_in_out_err([env, "-w", "-e", "exit"], "", [], /RUBY_GC_HEAP_INIT_SLOTS=100000/, "[ruby-core:39795]")
+    # Value of GC_HEAP_INIT_SLOTS is 10000
+    assert_in_out_err([env, "-w", "-e", "exit"], "", [], /\(default value: 10000\)/)
 
     env = {
       "RUBY_GC_HEAP_GROWTH_FACTOR" => "2.0",