[perl #70530] Calling multimethod causes segfault

[via RT]

# New Ticket Created by  Arne Skjærholt 
# Please include the string:  [perl #70530]
# in the subject line of all future correspondence about this issue. 
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=70530 >


This code snippet: class T does Associative { multi method
postcircumfix:<{ }> { }; }; my T $t .= new; say $t<a>; causes a
segfault on Rakudo on OS X 10.6.2.

Recompiling Rakudo with -O0 -g gives some interesting data in gdb:
278             GETATTR_P6LowLevelSig_elements(interp, signature,
sig_elem_info);

(gdb) p *signature
$8 = {
  flags = 266752,
  vtable = 0x10183d000,
  data = 0x0,
  _metadata = 0x0,
  _synchronize = 0x0
}

The culprit is on line 47 of src/pmc/p6_lowlevelsig.h, which calls
PMC_data(signature)->elements. Since signature->data is NULL, we get a
NULL pointer dereference.