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.