Fix problem exposed by gdb.server/stop-reply-no-thread-multi.exp

Fix problem exposed by gdb.server/stop-reply-no-thread-multi.exp

Running gdb.server/stop-reply-no-thread-multi.exp with "maint set
target-non-stop on" occasionally hit an internal error like this:

  ...
  continue
  Continuing.
  warning: multi-threaded target stopped without sending a thread-id, using first non-exited thread
  /home/pedro/gdb/binutils-gdb/src/gdb/inferior.c:291: internal-error: inferior* find_inferior_pid(process_stratum_target*, int): Assertion `pid != 0' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.

  This is a bug, please report it.
  FAIL: gdb.server/stop-reply-no-thread-multi.exp: to_disable=Tthread: continue until exit (GDB internal error)

The backtrace looks like this:

 ...
 #5  0x0000560357b0879c in internal_error (file=0x560357be6c18 "/home/pedro/gdb/binutils-gdb/src/gdb/inferior.c", line=291, fmt=0x560357be6b21 "%s: Assertion `%s' failed.") at /home/pedro/gdb/binutils-gdb/src/gdbsupport/errors.cc:55
 #6  0x000056035762061b in find_inferior_pid (targ=0x5603596e9560, pid=0) at /home/pedro/gdb/binutils-gdb/src/gdb/inferior.c:291
 #7  0x00005603576206e6 in find_inferior_ptid (targ=0x5603596e9560, ptid=...) at /home/pedro/gdb/binutils-gdb/src/gdb/inferior.c:305
 #8  0x00005603577d43ed in remote_target::check_pending_events_prevent_wildcard_vcont (this=0x5603596e9560, may_global_wildcard=0x7fff84fb05f0) at /home/pedro/gdb/binutils-gdb/src/gdb/remote.c:7215
 #9  0x00005603577d2a9c in remote_target::commit_resumed (this=0x5603596e9560) at /home/pedro/gdb/binutils-gdb/src/gdb/remote.c:6680
 ...

pid is 0 in this case because the queued event is a process exit event
with no pid associated:

 (top-gdb) p event->ws
 During symbol reading: .debug_line address at offset 0x563c9a is 0 [in module /home/pedro/gdb/binutils-gdb/build/gdb/gdb]
 $1 = {kind = TARGET_WAITKIND_EXITED, value = {integer = 0, sig = GDB_SIGNAL_0, related_pid = {m_pid = 0, m_lwp = 0, m_tid = 0}, execd_pathname = 0x0, syscall_number = 0}}
 (top-gdb)

This fixes it, and adds a "maint set target-non-stop on/off" axis to the testcase.

gdb/ChangeLog:

	* remote.c
	(remote_target::check_pending_events_prevent_wildcard_vcont):
	Check whether the event's ptid is not null_ptid before looking up
	the corresponding inferior.

gdb/testsuite/ChangeLog:

	* gdb.server/stop-reply-no-thread-multi.exp (run_test): Add
	"target_non_stop" parameter and use it.
	(top level): Add "maint set target-non-stop on/off" testing axis.

Change-Id: Ia30cf275305ee4dcbbd33f731534cd71d1550eaa