Convert fatal to gdb_assert in both i386_dr_low_set_addr

Convert fatal to gdb_assert in both i386_dr_low_set_addr

This commit converts if..fatal checks in both i386_dr_low_set_addr
implementations to gdb_asserts.  It's not obvious from the context,
but the conditional in both cases is changed to match the equivalent
conditional in the i386_dr_low_get_addr implementations.  Nothing
fundamental has changed because DR_FIRSTADDR is zero.  This commit
also removes a vague comment in Linux i386_dr_low_get_addr.  I could
have reworded the comment (and replicated it three times for the other
identical assertions) but I think the existence of specific functions
for the status and control registers makes it fairly obvious what is
going on.

gdb/gdbserver/ChangeLog:

	* linux-x86-low.c (i386_dr_low_set_addr): Replace check with
	gdb_assert.
	(i386_dr_low_get_addr): Remove vague comment.
	* win32-i386-low.c (i386_dr_low_set_addr): Replace check with
	gdb_assert.