Fix crash on optimized-out entry data values

Fix crash on optimized-out entry data values

The tests at
<https://sourceware.org/ml/gdb-patches/2014-07/msg00277.html> show
that comparing a fully optimized out value's contents with a value
that has not been optimized out, or is partially optimized out crashes
GDB:

 (gdb) bt
 #0  __memcmp_sse4_1 () at ../sysdeps/x86_64/multiarch/memcmp-sse4.S:816
 #1  0x00000000005a1914 in memcmp_with_bit_offsets (ptr1=0x202b2f0 "\n", offset1_bits=0, ptr2=0x0, offset2_bits=0, length_bits=32)
     at /home/pedro/gdb/mygit/build/../src/gdb/value.c:678
 #2  0x00000000005a1a05 in value_available_contents_bits_eq (val1=0x2361ad0, offset1=0, val2=0x23683b0, offset2=0, length=32)
     at /home/pedro/gdb/mygit/build/../src/gdb/value.c:717
 #3  0x00000000005a1c09 in value_available_contents_eq (val1=0x2361ad0, offset1=0, val2=0x23683b0, offset2=0, length=4)
     at /home/pedro/gdb/mygit/build/../src/gdb/value.c:769
 #4  0x00000000006033ed in read_frame_arg (sym=0x1b78d20, frame=0x19bca50, argp=0x7fff4aba82b0, entryargp=0x7fff4aba82d0)
     at /home/pedro/gdb/mygit/build/../src/gdb/stack.c:416
 #5  0x0000000000603abb in print_frame_args (func=0x1b78cb0, frame=0x19bca50, num=-1, stream=0x1aea450) at /home/pedro/gdb/mygit/build/../src/gdb/stack.c:671
 #6  0x0000000000604ae8 in print_frame (frame=0x19bca50, print_level=0, print_what=SRC_AND_LOC, print_args=1, sal=...)
     at /home/pedro/gdb/mygit/build/../src/gdb/stack.c:1205
 #7  0x0000000000604050 in print_frame_info (frame=0x19bca50, print_level=0, print_what=SRC_AND_LOC, print_args=1, set_current_sal=1)
     at /home/pedro/gdb/mygit/build/../src/gdb/stack.c:857
 #8  0x00000000006029b3 in print_stack_frame (frame=0x19bca50, print_level=0, print_what=SRC_AND_LOC, set_current_sal=1)
     at /home/pedro/gdb/mygit/build/../src/gdb/stack.c:169
 #9  0x00000000005fc4b8 in print_stop_event (ws=0x7fff4aba8790) at /home/pedro/gdb/mygit/build/../src/gdb/infrun.c:6068
 #10 0x00000000005fc830 in normal_stop () at /home/pedro/gdb/mygit/build/../src/gdb/infrun.c:6214

The 'ptr2=0x0' in frame #1 is val2->contents, and since git 4f14910f:

    gdb/ChangeLog
    2013-11-26  Andrew Burgess  <aburgess@broadcom.com>

        * value.c (allocate_optimized_out_value): Mark value as non-lazy.

... a fully optimized-out value can have it's value contents buffer
NULL.

As a spotgap fix, revert 4f14910f, with a comment.  A full fix would
be too invasive for 7.8.

gdb/
2014-07-22  Pedro Alves  <palves@redhat.com>

	* value.c (allocate_optimized_out_value): Don't mark value as
	non-lazy.