Fix coroutine support on win32

Ruby master branch currently fails on win32 MINGW at this spec:
https://github.com/ruby/spec/blob/master/core/thread/element_set_spec.rb

MINGW makes use of setjmp3() implemented in MSVCRT.DLL.
This function traverses the SEH list up to a terminating pointer 0xFFFFFFFF.
It therefore currently segfaults on NULL.
The SEH linked list must be terminated by 0xFFFFFFFF instead of NULL.

This fixes the issue mentioned here:
  https://github.com/ruby/ruby/pull/2279#issuecomment-509508810

coroutine/win32/Context.h

@@ -42,7 +42,7 @@ static inline void coroutine_initialize(
     *--context->stack_pointer = (void*)start;
 
     /* Windows Thread Information Block */
-    *--context->stack_pointer = 0; /* fs:[0] */
+    *--context->stack_pointer = (void*)0xFFFFFFFF; /* fs:[0] */
     *--context->stack_pointer = (void*)top; /* fs:[4] */
     *--context->stack_pointer = (void*)stack;  /* fs:[8] */