[PRISM] Fix st_table memory leak

Co-authored-by: Matthew Draper <matthew@trebex.net>

prism_compile.c

@@ -5079,7 +5079,7 @@ pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret,
             }
         }
 
-        free(index_lookup_table);
+        st_free_table(index_lookup_table);
 
         if (!PM_NODE_TYPE_P(scope_node->ast_node, PM_ENSURE_NODE)) {
             ADD_INSN(ret, &dummy_line_node, leave);