Avoid crash with GCC trunk

With GCC trunk, gdb.ada/access_to_packed_array.exp causes a GDB crash.
The problem is that ptype tries to resolve a dynamic type.  However,
the inferior is not running, so there are no frames.

This patch updates dwarf2_evaluate_loc_desc::get_frame_base to handle
this situation.

gdb/ChangeLog
2021-05-17  Tom Tromey  <tromey@adacore.com>

	* dwarf2/loc.c (dwarf2_evaluate_loc_desc::get_frame_base): Throw
	if frame is null.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2021-05-17  Tom Tromey  <tromey@adacore.com>
+
+	* dwarf2/loc.c (dwarf2_evaluate_loc_desc::get_frame_base): Throw
+	if frame is null.
+
 2021-05-17  Tom Tromey  <tromey@adacore.com>
 
 	* nat/linux-osdata.c (user_from_uid, time_from_time_t)

gdb/dwarf2/loc.c

@@ -785,6 +785,9 @@ public:
      its length in LENGTH.  */
   void get_frame_base (const gdb_byte **start, size_t * length) override
   {
+    if (frame == nullptr)
+      error (_("frame address is not available."));
+
     /* FIXME: cagney/2003-03-26: This code should be using
        get_frame_base_address(), and then implement a dwarf2 specific
        this_base method.  */