in traceback, handle the case where we've called through a nil function pointer

R=rsc
DELTA=7  (7 added, 0 deleted, 0 changed)
OCL=18372
CL=18372

src/runtime/rt2_amd64.c

@@ -25,6 +25,13 @@ traceback(uint8 *pc, uint8 *sp, void* r15)
 	// store local copy of per-process data block that we can write as we unwind
 	mcpy((byte*)&g, (byte*)r15, sizeof(G));
 
+	// if the PC is zero, it's probably due to a nil function pointer.
+	// pop the failed frame.
+	if(pc == nil) {
+		pc = ((uint8**)sp)[0];
+		sp += 8;
+	}
+
 	counter = 0;
 	name = "panic";
 	for(;;){