Assert that a floating type's length is at least as long as its format

Assert that a floating type's length is at least as long as its format

This would have caught the HP/PA bug fixed in the previous patch:

 .../src/gdb/gdbtypes.c:4690: internal-error: arch_float_type: Assertion `len >= floatformat_totalsize_bytes (floatformats[0])' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n)

Tested on x86-64 Fedora 23, --enable-targets=all.

gdb/ChangeLog:
2016-03-09  Pedro Alves  <palves@redhat.com>

	* doublest.c (floatformat_totalsize_bytes): New function.
	(floatformat_from_type): Assert that the type's length is at least
	as long as the floatformat's totalsize.
	* doublest.h (floatformat_totalsize_bytes): New declaration.
	* gdbtypes.c (arch_float_type): Assert that the type's length is
	at least as long as the floatformat's totalsize.