Add asserts in target_fetch/store_registers

Add asserts in target_fetch/store_registers

We are currently assuming that regcache->ptid is equal to inferior_ptid
when we call target_fetch/store_registers.  These asserts just validate
that assumption.  Also, since the following patches will change target
code to use regcache->ptid instead of inferior_ptid, asserting that they
are the same should ensure that our changes don't have any unintended
consequences.

gdb/ChangeLog:

	* target.c (target_fetch_registers, target_store_registers): Add
	assert.