[PRISM] Suppress compiler warnings

../prism_compile.c: In function ‘pm_compile_node’:
../compile.c:583:24: warning: ‘retry_end_l’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  583 |     anchor->last->next = elem;
      |     ~~~~~~~~~~~~~~~~~~~^~~~~~
In file included from ../compile.c:14256:
../prism_compile.c:5796:16: note: ‘retry_end_l’ was declared here
 5796 |         LABEL *retry_end_l;
      |                ^~~~~~~~~~~
../compile.c:255:42: warning: ‘retry_label’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  255 | #define LABEL_REF(label) ((label)->refcnt++)
      |                                          ^~
In file included from ../compile.c:14256:
../prism_compile.c:5795:16: note: ‘retry_label’ was declared here
 5795 |         LABEL *retry_label;
      |                ^~~~~~~~~~~
../prism_compile.c:5919:52: warning: ‘previous_block’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 5919 |             ISEQ_COMPILE_DATA(iseq)->current_block = previous_block;
      |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~

prism_compile.c

@@ -5791,9 +5791,9 @@ pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret,
         const pm_forwarding_super_node_t *cast = (const pm_forwarding_super_node_t *) node;
         const rb_iseq_t *block = NULL;
 
-        const rb_iseq_t *previous_block;
-        LABEL *retry_label;
-        LABEL *retry_end_l;
+        const rb_iseq_t *previous_block = NULL;
+        LABEL *retry_label = NULL;
+        LABEL *retry_end_l = NULL;
 
         if (cast->block != NULL) {
             previous_block = ISEQ_COMPILE_DATA(iseq)->current_block;