[gdb] Fix segv in "maint print symbols" for ada exec

When using the executable from test-case gdb.ada/access_to_packed_array.exp
(read-in using -readnow) and printing the symbols using "maint print symbols",
we run into a segv:
...
$ gdb -readnow -batch access_to_packed_array/foo -ex "maint print symbols"
   ...
     info: array (<>) of character; computed at runtime
     ptr: range 0 .. 2147483647; computed at runtime
Aborted (core dumped)
...

What happens is that dwarf2_evaluate_property gets called and sets the local
frame variable to the current frame, which happens to be NULL.  Subsequently
the PROP_LOCLIST handling code is executed, where get_frame_address_in_block
gets called with argument NULL, and the segv is triggered.

Fix this by handling a NULL frame in the PROP_LOCLIST handling code in
dwarf2_evaluate_property.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* dwarf2/loc.c (dwarf2_evaluate_property): Handle NULL frame in
	PROP_LOCLIST handling code.

gdb/testsuite/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* gdb.ada/access_to_packed_array.exp: Test printing of expanded
	symtabs.

1 files changed, 5 insertions, 1 deletions

diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index a5074da..5155cff 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -2511,11 +2511,15 @@ dwarf2_evaluate_property (const struct dynamic_prop *prop,
       {
 	struct dwarf2_property_baton *baton
 	  = (struct dwarf2_property_baton *) prop->data.baton;
-	CORE_ADDR pc = get_frame_address_in_block (frame);
+	CORE_ADDR pc;
 	const gdb_byte *data;
 	struct value *val;
 	size_t size;
 
+	if (frame == NULL
+	    || !get_frame_address_in_block_if_available (frame, &pc))
+	  return false;
+
 	data = dwarf2_find_location_expression (&baton->loclist, &size, pc);
 	if (data != NULL)
 	  {