Thread start bug on ppc64

[Douglas Katzman]

::: Running :GC-DEADLOCK
....fatal error encountered in SBCL pid 49436 tid 54974:
GC invariant lost, file "thread.c", line 565

which is:
gc_assert(functionp(startfun));

[Douglas Katzman (dougk)]

With some printf() debugging:

.......fatal error encountered in SBCL pid 8073 tid 18192:
bad startup info: 0x10035f0020 (nil)

--- a/src/runtime/thread.c
+++ b/src/runtime/thread.c
@@ -562,6 +562,7 @@ void* new_thread_trampoline(void* arg)
     struct vector* startup_info = VECTOR(lispthread->startup_info); // 'lispthread' is pinned
     gc_assert(header_widetag(startup_info->header) == SIMPLE_VECTOR_WIDETAG);
     lispobj startfun = startup_info->data[0]; // 'startup_info' is pinned
+ if (!functionp(startfun)) lose("bad startup info: %p %p\n", startup_info, startfun);
     gc_assert(functionp(startfun));

How is this possible?