Fix floating conversion buffer overrun when host/target format matches

Fix floating conversion buffer overrun when host/target format matches

Running the testsuite with a gdb configured with --enable-libmcheck
reveals a problem:

  (gdb) ptype 3 * 2.0
  type = <12-byte float>
  memory clobbered past end of allocated block
  ERROR: Process no longer exists
  UNRESOLVED: gdb.ada/ptype_arith_binop.exp: ptype 3 * 2.0

  (gdb) PASS: gdb.dlang/expression.exp: ptype 0x1.FFFFFFFFFFFFFp1023
  ptype 0x1p-52L
  type = real
  memory clobbered past end of allocated block
  ERROR: Process no longer exists
  UNRESOLVED: gdb.dlang/expression.exp: ptype 0x1p-52L

Even though this shows up with Ada and D, it's easy to reproduce in C
too.  We just need to print a long double, when the current arch is
32-bit, which is the default when gdb starts up:

 $ ./gdb -q -ex "ptype 1.0L"
 type = long double
 memory clobbered past end of allocated block
 Aborted (core dumped)

Valgrind shows:

 ==22159== Invalid write of size 8
 ==22159==    at 0x8464A9: floatformat_from_doublest (doublest.c:756)
 ==22159==    by 0x846822: store_typed_floating (doublest.c:867)
 ==22159==    by 0x6A7959: value_from_double (value.c:3662)
 ==22159==    by 0x6A9F2D: evaluate_subexp_standard (eval.c:745)
 ==22159==    by 0x7F31AF: evaluate_subexp_c (c-lang.c:716)
 ==22159==    by 0x6A8986: evaluate_subexp (eval.c:79)
 ==22159==    by 0x6A8BA3: evaluate_type (eval.c:174)
 ==22159==    by 0x817CCF: whatis_exp (typeprint.c:456)
 ==22159==    by 0x817EAA: ptype_command (typeprint.c:508)
 ==22159==    by 0x5F267B: do_cfunc (cli-decode.c:105)
 ==22159==    by 0x5F5618: cmd_func (cli-decode.c:1885)
 ==22159==    by 0x83622A: execute_command (top.c:475)
 ==22159==  Address 0x8c6cb28 is 8 bytes inside a block of size 12 alloc'd
 ==22159==    at 0x4C2AA98: calloc (vg_replace_malloc.c:711)
 ==22159==    by 0x87384A: xcalloc (common-utils.c:83)
 ==22159==    by 0x873889: xzalloc (common-utils.c:93)
 ==22159==    by 0x6A34CB: allocate_value_contents (value.c:1036)
 ==22159==    by 0x6A3501: allocate_value (value.c:1047)
 ==22159==    by 0x6A790A: value_from_double (value.c:3656)
 ==22159==    by 0x6A9F2D: evaluate_subexp_standard (eval.c:745)
 ==22159==    by 0x7F31AF: evaluate_subexp_c (c-lang.c:716)
 ==22159==    by 0x6A8986: evaluate_subexp (eval.c:79)
 ==22159==    by 0x6A8BA3: evaluate_type (eval.c:174)
 ==22159==    by 0x817CCF: whatis_exp (typeprint.c:456)
 ==22159==    by 0x817EAA: ptype_command (typeprint.c:508)
 ==22159==
 type = long double
 (gdb)

Even if the target and host floating-point formats match, the length
of the types might still be different.  On x86, long double is the
80-bit extended precision type on both 32-bit and 64-bit ABIs, but by
default it is stored as 12 bytes on 32-bit, and 16 bytes on 64-bit,
for alignment reasons.  Several places in doublest.c already consider
this, but floatformat_to_doublest and floatformat_from_doublest miss
it.  E.g., convert_typed_floating and store_typed_floating,

Tested on x86-64 Fedora 23 with --enable-libmcheck, where it fixes the
crashed above.

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

	* doublest.c: Extend comments.
	(floatformat_to_doublest, floatformat_from_doublest): Copy the
	floatformat's total size, not the host type's size.