Fix any_thread_of_inferior

Running gdb-term.exp against gdbserver with "maint set target-non-stop
on", runs into this:

  [infrun] fetch_inferior_event: exit
  [infrun] fetch_inferior_event: enter
  /home/pedro/gdb/binutils-gdb/src/gdb/thread.c:72: internal-error: thread_info* inferior_thread(): Assertion `current_thread_ != nullptr' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.

  This is a bug, please report it.  For instructions, see:
  <https://www.gnu.org/software/gdb/bugs/>.

  FAIL: gdb.base/gdb-sigterm.exp: expect eof #2 (GDB internal error)
  Resyncing due to internal error.
  ERROR: : spawn id exp9 not open
      while executing
  "expect {
  -i exp9 -timeout 10
	      -re "Quit this debugging session\\? \\(y or n\\) $" {
		  send_gdb "n\n" answer
		  incr count
	      }
	      -re "Create ..."
      ("uplevel" body line 1)
      invoked from within
  "uplevel $body" NONE : spawn id exp9 not open
  ERROR: Could not resync from internal error (timeout)
  gdb.base/gdb-sigterm.exp: expect eof #2: stepped 0 times
  UNRESOLVED: gdb.base/gdb-sigterm.exp: 50 SIGTERM passes

The assertion fails here:

  ...
  #5  0x000055af4b4a7164 in internal_error (file=0x55af4b5e5de8 "/home/pedro/gdb/binutils-gdb/src/gdb/thread.c", line=72, fmt=0x55af4b5e5ce9 "%s: Assertion `%s' failed.") at /home/pedro/gdb/binutils-gdb/src/gdbsupport/errors.cc:55
  #6  0x000055af4b25fc43 in inferior_thread () at /home/pedro/gdb/binutils-gdb/src/gdb/thread.c:72
  #7  0x000055af4b26177e in any_thread_of_inferior (inf=0x55af4cf874f0) at /home/pedro/gdb/binutils-gdb/src/gdb/thread.c:638
  #8  0x000055af4b26eec8 in kill_or_detach (inf=0x55af4cf874f0, from_tty=0) at /home/pedro/gdb/binutils-gdb/src/gdb/top.c:1665
  #9  0x000055af4b26f37f in quit_force (exit_arg=0x0, from_tty=0) at /home/pedro/gdb/binutils-gdb/src/gdb/top.c:1767
  #10 0x000055af4b2f72a7 in quit () at /home/pedro/gdb/binutils-gdb/src/gdb/utils.c:633
  #11 0x000055af4b2f730b in maybe_quit () at /home/pedro/gdb/binutils-gdb/src/gdb/utils.c:657
  #12 0x000055af4b1adb74 in ser_base_wait_for (scb=0x55af4d02e460, timeout=0) at /home/pedro/gdb/binutils-gdb/src/gdb/ser-base.c:236
  #13 0x000055af4b1adf0f in do_ser_base_readchar (scb=0x55af4d02e460, timeout=0) at /home/pedro/gdb/binutils-gdb/src/gdb/ser-base.c:365
  #14 0x000055af4b1ae06d in generic_readchar (scb=0x55af4d02e460, timeout=0, do_readchar=0x55af4b1adeb1 <do_ser_base_readchar(serial*, int)>) at /home/pedro/gdb/binutils-gdb/src/gdb/ser-base.c:444
  ...

The bug is that any_thread_of_inferior incorrectly assumes that
there's always a selected thread.  This fixes it.

gdb/ChangeLog:

	* thread.c (any_thread_of_inferior): Check if there's a selected
	thread before calling inferior_thread().

Change-Id: Ica4b9ec746121a7a7c22bef09baea72103b3853d

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2021-03-19  Pedro Alves  <[email protected]>
+
+	* thread.c (any_thread_of_inferior): Check if there's a selected
+	thread before calling inferior_thread().
+
 2021-03-18  Tom Tromey  <[email protected]>
 
 	* dwarf2/stringify.c (dwarf_unit_type_name): New function.  Use

gdb/thread.c

@@ -637,8 +637,8 @@ any_thread_of_inferior (inferior *inf)
 {
   gdb_assert (inf->pid != 0);
 
-  /* Prefer the current thread.  */
-  if (inf == current_inferior ())
+  /* Prefer the current thread, if there's one.  */
+  if (inf == current_inferior () && inferior_ptid != null_ptid)
     return inferior_thread ();
 
   for (thread_info *tp : inf->non_exited_threads ())