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

[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.