alpha-tdep.c/mips-tdep.c: "set heuristic-fence-post" is signed/zinteger.

alpha-tdep.c/mips-tdep.c: "set heuristic-fence-post" is signed/zinteger.

These commands are currently var_zinteger, hence their control
variable is signed.

gdb/
2013-04-19  Pedro Alves  <palves@redhat.com>

	* alpha-tdep.c (heuristic_fence_post): Change type to int.
	(alpha_heuristic_proc_start): Adjust to check -1 instead of
	UINT_MAX.
	* mips-tdep.c (heuristic_fence_post): Change type to int.
	(heuristic_proc_start): Adjust to check -1 instead of UINT_MAX.