Fix gdbserver --debug issues caught by Valgrind

Fix gdbserver --debug issues caught by Valgrind

Running gdbserver --debug under Valgrind shows:

 ==4803== Invalid read of size 4
 ==4803==    at 0x432B62: linux_write_memory (linux-low.c:5320)
 ==4803==    by 0x4143F7: write_inferior_memory (target.c:83)
 ==4803==    by 0x415895: remove_memory_breakpoint (mem-break.c:362)
 ==4803==    by 0x432EF5: linux_remove_point (linux-low.c:5460)
 ==4803==    by 0x416319: delete_raw_breakpoint (mem-break.c:802)
 ==4803==    by 0x4163F3: release_breakpoint (mem-break.c:842)
 ==4803==    by 0x416477: delete_breakpoint_1 (mem-break.c:869)
 ==4803==    by 0x4164EF: delete_breakpoint (mem-break.c:891)
 ==4803==    by 0x416843: delete_gdb_breakpoint_1 (mem-break.c:1069)
 ==4803==    by 0x4168D8: delete_gdb_breakpoint (mem-break.c:1098)
 ==4803==    by 0x4134E3: process_serial_event (server.c:4051)
 ==4803==    by 0x4138E4: handle_serial_event (server.c:4196)
 ==4803==  Address 0x4c6b930 is 0 bytes inside a block of size 1 alloc'd
 ==4803==    at 0x4A0645D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
 ==4803==    by 0x4240C6: xmalloc (common-utils.c:43)
 ==4803==    by 0x41439C: write_inferior_memory (target.c:80)
 ==4803==    by 0x415895: remove_memory_breakpoint (mem-break.c:362)
 ==4803==    by 0x432EF5: linux_remove_point (linux-low.c:5460)
 ==4803==    by 0x416319: delete_raw_breakpoint (mem-break.c:802)
 ==4803==    by 0x4163F3: release_breakpoint (mem-break.c:842)
 ==4803==    by 0x416477: delete_breakpoint_1 (mem-break.c:869)
 ==4803==    by 0x4164EF: delete_breakpoint (mem-break.c:891)
 ==4803==    by 0x416843: delete_gdb_breakpoint_1 (mem-break.c:1069)
 ==4803==    by 0x4168D8: delete_gdb_breakpoint (mem-break.c:1098)
 ==4803==    by 0x4134E3: process_serial_event (server.c:4051)
 ==4803==

And:

 ==7272== Conditional jump or move depends on uninitialised value(s)
 ==7272==    at 0x3615E48361: vfprintf (vfprintf.c:1634)
 ==7272==    by 0x414E89: debug_vprintf (debug.c:60)
 ==7272==    by 0x42800A: debug_printf (common-debug.c:35)
 ==7272==    by 0x43937B: my_waitpid (linux-waitpid.c:149)
 ==7272==    by 0x42D740: linux_wait_for_event_filtered (linux-low.c:2441)
 ==7272==    by 0x42DADA: linux_wait_for_event (linux-low.c:2552)
 ==7272==    by 0x42E165: linux_wait_1 (linux-low.c:2860)
 ==7272==    by 0x42F5D8: linux_wait (linux-low.c:3453)
 ==7272==    by 0x4144A4: mywait (target.c:107)
 ==7272==    by 0x413969: handle_target_event (server.c:4214)
 ==7272==    by 0x41A1A6: handle_file_event (event-loop.c:429)
 ==7272==    by 0x41996D: process_event (event-loop.c:184)

gdb/ChangeLog:
2015-08-06  Pedro Alves  <palves@redhat.com>

	* nat/linux-waitpid.c (my_waitpid): Only print *status if waitpid
	returned > 0.

gdb/gdbserver/ChangeLog:
2015-08-06  Pedro Alves  <palves@redhat.com>

	* linux-low.c (linux_write_memory): Rewrite debug output to avoid
	reading beyond the passed in buffer length.