[PR tui/14880] Fetch values before comparing their contents.

PR tui/14880 shows a reproducer that triggers this assertion:

  int
  value_available_contents_eq (const struct value *val1, int offset1,
  			     const struct value *val2, int offset2,
  			     int length)
  {
    int idx1 = 0, idx2 = 0;

    /* This routine is used by printing routines, where we should
       already have read the value.  Note that we only know whether a
       value chunk is available if we've tried to read it.  */
    gdb_assert (!val1->lazy && !val2->lazy);

(top-gdb) bt
#0  internal_error (file=0x88a26c "../../src/gdb/value.c", line=549, string=0x88a220 "%s: Assertion `%s' failed.") at ../../src/gdb/utils.c:844
#1  0x000000000057b9cd in value_available_contents_eq (val1=0x10fa900, offset1=0, val2=0x10f9e10, offset2=0, length=8) at ../../src/gdb/value.c:549
#2  0x00000000004fd756 in tui_get_register (frame=0xd5c430, data=0x109a548, regnum=0, changedp=0x109a560) at ../../src/gdb/tui/tui-regs.c:736
#3  0x00000000004fd111 in tui_check_register_values (frame=0xd5c430) at ../../src/gdb/tui/tui-regs.c:521
#4  0x0000000000501884 in tui_check_data_values (frame=0xd5c430) at ../../src/gdb/tui/tui-windata.c:234
#5  0x00000000004f976f in tui_selected_frame_level_changed_hook (level=1) at ../../src/gdb/tui/tui-hooks.c:222
#6  0x00000000006f0681 in select_frame (fi=0xd5c430) at ../../src/gdb/frame.c:1490
#7  0x00000000005dd94b in up_silently_base (count_exp=0x0) at ../../src/gdb/stack.c:2268
#8  0x00000000005dd985 in up_command (count_exp=0x0, from_tty=1) at ../../src/gdb/stack.c:2280
#9  0x00000000004dc5cf in do_cfunc (c=0xd3f720, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:113
#10 0x00000000004df664 in cmd_func (cmd=0xd3f720, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:1888
#11 0x00000000006e43e1 in execute_command (p=0xc7e6c2 "", from_tty=1) at ../../src/gdb/top.c:489

The fix is to fetch the value before comparing the contents.  The
comment additions to value.h explain why it can't be
value_available_contents_eq itself that fetches the contents.

Tested on x86_64 Fedora 17.

gdb/
2013-06-28  Pedro Alves  <palves@redhat.com>

	PR tui/14880
	* tui/tui-regs.c (tui_get_register): Fetch register value contents
	before checking whether they're available.
	* value.c (value_available_contents_eq): Change comment.
	* value.h (value_available_contents_eq): Expand comment.
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