fix SEGV touching uninitialized memory

This function can be called from boot_defclass().
No assumption can be made about object internals.

(lldb) run
Process 2386 launched: './miniruby' (x86_64)
Process 2386 stopped
* thread #1: tid = 0x13f3b6, 0x00000001001e0b26 miniruby`rb_class_path_cached(klass=4311373720) + 20 at variable.c:321, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x8)
    frame #0: 0x00000001001e0b26 miniruby`rb_class_path_cached(klass=4311373720) + 20 at variable.c:321
   318  VALUE
   319  rb_class_path_cached(VALUE klass)
   320  {
-> 321      st_table *ivtbl = RCLASS_IV_TBL(klass);
   322      st_data_t n;
   323
   324      if (!ivtbl) return Qnil;
(lldb) bt
* thread #1: tid = 0x13f3b6, 0x00000001001e0b26 miniruby`rb_class_path_cached(klass=4311373720) + 20 at variable.c:321, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x8)
  * frame #0: 0x00000001001e0b26 miniruby`rb_class_path_cached(klass=4311373720) + 20 at variable.c:321
    frame #1: 0x000000010009cbd0 miniruby`rb_raw_obj_info(buff="0x0000000100fa5798 [2    ] T_CLASS", buff_size=256, obj=4311373720) + 1393 at gc.c:9341
    frame #2: 0x000000010009cf16 miniruby`obj_info(obj=4311373720) + 98 at gc.c:9423
    frame #3: 0x000000010008ca87 miniruby`newobj_init(klass=0, flags=66, v1=0, v2=0, v3=0, wb_protected=1, objspace=0x00000001007cf280, obj=4311373720) + 338 at gc.c:1887
    frame #4: 0x000000010008cd51 miniruby`newobj_of(klass=0, flags=66, v1=0, v2=0, v3=0, wb_protected=1) + 171 at gc.c:1970
    frame #5: 0x000000010008ce1b miniruby`rb_wb_protected_newobj_of(klass=0, flags=66) + 54 at gc.c:1990
    frame #6: 0x0000000100027563 miniruby`class_alloc(flags=2, klass=0) + 46 at class.c:165
    frame #7: 0x000000010002761a miniruby`rb_class_boot(super=0) + 35 at class.c:203
    frame #8: 0x0000000100028612 miniruby`boot_defclass(name="BasicObject", super=0) + 28 at class.c:537
    frame #9: 0x000000010002868b miniruby`Init_class_hierarchy + 26 at class.c:548
    frame #10: 0x00000001000efe69 miniruby`InitVM_Object + 9 at object.c:3892
    frame #11: 0x00000001000f138e miniruby`Init_Object + 57 at object.c:4122
    frame #12: 0x00000001000a59bd miniruby`rb_call_inits + 29 at inits.c:23
    frame #13: 0x000000010007af30 miniruby`ruby_setup + 229 at eval.c:61
    frame #14: 0x000000010007af7e miniruby`ruby_init + 13 at eval.c:78
    frame #15: 0x0000000100000c58 miniruby`main(argc=2, argv=0x00007fff5fbfdbf0) + 88 at main.c:41
    frame #16: 0x00007fff88eda5ad libdyld.dylib`start + 1
(lldb)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61562 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: shyouhei

variable.c

@@ -318,10 +318,11 @@ rb_class_path_no_cache(VALUE klass)
 VALUE
 rb_class_path_cached(VALUE klass)
 {
-    st_table *ivtbl = RCLASS_IV_TBL(klass);
+    st_table *ivtbl;
     st_data_t n;
 
-    if (!ivtbl) return Qnil;
+    if (!RCLASS_EXT(klass)) return Qnil;
+    if (!(ivtbl = RCLASS_IV_TBL(klass))) return Qnil;
     if (st_lookup(ivtbl, (st_data_t)classpath, &n)) return (VALUE)n;
     if (st_lookup(ivtbl, (st_data_t)tmp_classpath, &n)) return (VALUE)n;
     return Qnil;