Guard against frame.c destructors running before frame-info.c's

On x86_64-windows, since 04e2ac7, we observe this internal error:

  [...]/gdbsupport/intrusive_list.h:458: internal-error: erase_element:
  Assertion `elem_node->prev != INTRUSIVE_LIST_UNLINKED_VALUE' failed.

Breaking in the destructors for intrusive_list and frame_info_ptr shows that
in this configuration, the destructors for frame.c's statically-stored
objects are run before frame-info.c's:

  Thread 1 hit Breakpoint 7, intrusive_list<frame_info_ptr,
  intrusive_base_node<frame_info_ptr> >::~intrusive_list (this=0x7ff69c418c90
  <frame_info_ptr::frame_list>, __in_chrg=<optimized out>)
  [...]/../gdbsupport/intrusive_list.h:250
  250	    clear ();
  (gdb) bt
  #0  intrusive_list<frame_info_ptr, intrusive_base_node<frame_info_ptr> >
      ::~intrusive_list (this=0x7ff69c418c90 <frame_info_ptr::frame_list>,
      __in_chrg=<optimized out>) [...]/../gdbsupport/intrusive_list.h:250
  #1  0x00007ff69b78edba in __tcf_1 () [...]/frame-info.c:27
  #2  0x00007ff9c457aa9f in msvcrt!_initterm_e ()
      from C:\Windows\System32\msvcrt.dll
  #3  0x00007ff69b8246a6 in captured_main_1 (context=0x5ffe00)
      [...]/main.c:1111
  bminor#4  0x00007ff69b825149 in captured_main (data=0x5ffe00) [...]/main.c:1320
  bminor#5  0x00007ff69b8251b1 in gdb_main (args=0x5ffe00) [...]/main.c:1345
  bminor#6  0x00007ff69b5d1730 in main (argc=2, argv=0x751730) [...]/gdb.c:32
  (gdb) c
  Continuing.

  Thread 1 hit Breakpoint 8, frame_info_ptr::~frame_info_ptr
  (this=0x7ff69c418e20 <selected_frame>, __in_chrg=<optimized out>)
  [...]/frame-info.h:74
  74	    if (is_linked ())
  (gdb) bt
  #0  frame_info_ptr::~frame_info_ptr (this=0x7ff69c418e20 <selected_frame>,
      __in_chrg=<optimized out>) [...]/frame-info.h:74
  #1  0x00007ff69b79a643 in __tcf_1 () [...]/frame.c:1675
  #2  0x00007ff9c457aa9f in msvcrt!_initterm_e () from
      C:\Windows\System32\msvcrt.dll
  #3  0x00007ff69b8246a6 in captured_main_1 (context=0x5ffe00)
      [...]/main.c:1111
  bminor#4  0x00007ff69b825149 in captured_main (data=0x5ffe00) [...]/main.c:1320
  bminor#5  0x00007ff69b8251b1 in gdb_main (args=0x5ffe00) [...]/main.c:1345
  bminor#6  0x00007ff69b5d1730 in main (argc=2, argv=0x751730) [...]/gdb.c:32

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/frame-info.h

@@ -76,7 +76,11 @@ class frame_info_ptr : public intrusive_list_node<frame_info_ptr>
 
   ~frame_info_ptr ()
   {
-    frame_list.erase (frame_list.iterator_to (*this));
+    /* If this node has static storage, it may be deleted after
+       frame_list.  Attempting to erase ourselves would then trigger
+       internal errors, so make sure we are still linked first.  */
+    if (is_linked ())
+      frame_list.erase (frame_list.iterator_to (*this));
   }
 
   frame_info_ptr &operator= (const frame_info_ptr &other)