Don't try to get the TIB address without an inferior

Don't try to get the TIB address without an inferior

The target_get_tib_address call always fails in this case, and there is an
error when changing the program with the file command:

(gdb) file allocer64.exe
Reading symbols from allocer64.exe...
You can't do that when your target is `exec'

Now it will skip this part, there is no need to rebase the executable without
an inferior anyways.

gdb/ChangeLog:

2020-03-06  Hannes Domani  <ssbssa@yahoo.de>

	* windows-tdep.c (windows_solib_create_inferior_hook):
	Check if inferior is running.