Fix GDB crash in dprintf.exp

Fix GDB crash in dprintf.exp

I see GDB crashes in dprintf.exp on aarch64-linux testing,

(gdb) PASS: gdb.base/dprintf.exp: agent: break 29
set dprintf-style agent^M
(gdb) PASS: gdb.base/dprintf.exp: agent: set dprintf style to agent
continue^M
Continuing.
ASAN:SIGSEGV
=================================================================
==22475==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000008 (pc 0x000000494820 sp 0x7fff389b83a0 bp 0x62d000082417 T0)
    #0 0x49481f in remote_add_target_side_commands /home/yao/SourceCode/gnu/gdb/git/gdb/remote.c:9190^M
    #1 0x49e576 in remote_add_target_side_commands /home/yao/SourceCode/gnu/gdb/git/gdb/remote.c:9174^M
    #2 0x49e576 in remote_insert_breakpoint /home/yao/SourceCode/gnu/gdb/git/gdb/remote.c:9240^M
    #3 0x5278b7 in insert_bp_location /home/yao/SourceCode/gnu/gdb/git/gdb/breakpoint.c:2734^M
    #4 0x52ac09 in insert_breakpoint_locations /home/yao/SourceCode/gnu/gdb/git/gdb/breakpoint.c:3159^M
    #5 0x52ac09 in update_global_location_list /home/yao/SourceCode/gnu/gdb/git/gdb/breakpoint.c:12686

the root cause of this problem in this case is about linespec and
symtab which produces additional incorrect location and a NULL is added to
bp_tgt->tcommands.  I posted a patch
https://sourceware.org/ml/gdb-patches/2015-12/msg00321.html to fix it
in linespec (the fix causes regression), but GDB still shouldn't add
NULL into bp_tgt->tcommands.  The logic of build_target_command_list
looks odd to me.  If we get something wrong in parse_cmd_to_aexpr (it
returns NULL), we shouldn't continue, instead we should set flag
null_command_or_parse_error.  This is what this patch does.  In the
meantime, we find build_target_condition_list has the same problem, so
fix it too.

gdb:

2016-01-28  Yao Qi  <yao.qi@linaro.org>

	* breakpoint.c (build_target_command_list): Don't call continue
	if aexpr is NULL.
	(build_target_condition_list): Likewise.