Fix crash when breakpoint condition causes inferior exit

When using a breakpoint condition that causes an inferior exit, gdb
crashes with a null pointer access:

(gdb) b main if callexit()
Breakpoint 1 at 0x114b: file callexit.c, line 32.
(gdb) r
Starting program: /home/src/lappy/binutils-gdb.git/gdb/testsuite/gdb.base/callexit
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".
[Inferior 1 (process 218586) exited normally]
../../gdb/infcall.c:895:50: runtime error: member call on null pointer of type 'struct thread_fsm'
../../gdb/infcall.c:895:50: runtime error: member access within null pointer of type 'struct thread_fsm'

Fix this by checking the thread_fsm pointer beforehand, now the result
looks like this:

(gdb) b main if callexit()
Breakpoint 1 at 0x114b: file callexit.c, line 32.
(gdb) r
Starting program: /home/src/lappy/binutils-gdb.git/gdb/testsuite/gdb.base/callexit
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/../lib/libthread_db.so.1".
[Inferior 1 (process 220707) exited normally]
❌ Error in testing condition for breakpoint 1:
The program being debugged exited while in a function called from GDB.
Evaluation of the expression containing the function
(callexit) will be abandoned.
❌ No registers.
(gdb)

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16156
Approved-By: Andrew Burgess <aburgess@redhat.com>