gdb/guile: don't try to print location for watchpoints

Currently, using the guile API, if a user tries to print a breakpoint
object that represents a watchpoint, then GDB will crash.  For
example:

  (gdb) guile (use-modules (gdb))
  (gdb) guile (define wp1 (make-breakpoint "some_variable" #:type BP_WATCHPOINT #:wp-class WP_WRITE))
  (gdb) guile (register-breakpoint! wp1)
  (gdb) guile (display wp1) (newline)
  Aborted (core dumped)

This turns out to be because GDB calls event_location_to_string on the
breakpoints location, and watchpoint breakpoints don't have a
location.

This commit resolves the crash by just skipping the printing of the
location if the breakpoint doesn't have one.

Potentially, we could improve on this by printing details about what
the watchpoint is watching, however, I'm considering this a possible
future enhancement, this commit focuses just on having GDB not crash.

gdb/ChangeLog:

	* guile/scm-breakpoint.c (bpscm_print_breakpoint_smob): Only print
	breakpoint locations when the breakpoint actually has a location.

gdb/testsuite/ChangeLog:

	* gdb.guile/scm-breakpoint.exp (test_watchpoints): Print the
	watchpoint object before and after registering it with GDB.