gdbserver crash if the_target->supports_z_point_type is NULL

When debugging on LynxOS targets (and probably on SPU targets as well),
inserting a breakpoint and resuming the program's execution causes
GDBserver to crash.

The crash occurs while handling the Z0 packet sent by GDB to insert
our breakpoint, because z_type_supported calls
the_target->supports_z_point_type without checking that it is not NULL
This patch fixes the issue by making z_type_supported return false if
the_target->supports_z_point_type is NULL.

gdb/gdbserver/ChangeLog:

        PR server/17023
        * mem-break.c (z_type_supported): Return zero if
        THE_TARGET->SUPPORTS_Z_POINT_TYPE is NULL.

Tested on ppx-lynx5.