gdb, gdbserver: fix read/write_ptid types

gdb, gdbserver: fix read/write_ptid types

In write_ptid(), a ptid's LWP member, which is declared long, is stored in
an int local variable before printing, potentially truncating it.  Fix it.

In read_ptid(), both PID and LWP are read as ULONGEST and then cast to
their respective type without checking for overflows.  Fix it.

In read_ptid(), an empty component is treated as zero.  Diagnose that as
an error, instead.

Approved-By: Tom Tromey <tom@tromey.com>