Cygwin: exec: don't access cygheap before it's initialized

This is a long-standing thinko.

When you exec a process, dll_crt0_0 in the child process calls
child_info_spawn::handle_spawn().  handle_spawn() initialises the
cygheap.

Now consider calling strace.  Strace is a non-Cygwin process dynamically
loading cygwin1.dll via LoadLibrary.  This in turn initializes the DLL:

- dll_crt0_0 finds that the process it attaches to has been exec'd, so
  child_info_spawn::handle_spawn() is called.

- If the DLL is being dynamically loaded, handle_spawn() calls
  child_info_spawn::get_parent_handle().  This in turn tries to set
  the moreinfo->myself_pinfo value inside the cygheap to NULL.

- However, at this time, the cygheap has not yet been initialized.  This
  only occurs in the cygheap_fixup_in_child() call after get_parent_handle()
  returns.

--> SEGV

This thinko never had a negative side effect, because the cygheap was
pre-allocated at DLL load time until commit 2f9b8ff00c ("Cygwin:
decouple cygheap from Cygwin DLL").  With 2f9b8ff00c, the cygheap
actually doesn't exist until after the call to cygheap_fixup_in_child().

Fix this problem by moving the assignment after the call to
cygheap_fixup_in_child().

Fixes: 3de7be4c1d ("* DevNotes: Add entry cgf-000007. [...]")
Fixes: 2f9b8ff00c ("Cygwin: decouple cygheap from Cygwin DLL")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/dcrt0.cc

@@ -620,7 +620,6 @@ bool
 child_info_spawn::get_parent_handle ()
 {
   parent = OpenProcess (PROCESS_VM_READ, FALSE, parent_winpid);
-  moreinfo->myself_pinfo = NULL;
   return !!parent;
 }
 
@@ -632,6 +631,8 @@ child_info_spawn::handle_spawn ()
   if (!dynamically_loaded || get_parent_handle ())
       {
 	cygheap_fixup_in_child (true);
+	if (dynamically_loaded)
+	  moreinfo->myself_pinfo = NULL;
 	memory_init ();
       }