Fix PR gdb/20295: GDB segfaults printing bitfield member of optimized out value

Fix PR gdb/20295: GDB segfaults printing bitfield member of optimized out value

With something like:

  struct A { int bitfield:4; } var;

If 'var' ends up wholly-optimized out, printing 'var.bitfield' crashes
gdb here:

 (top-gdb) bt
 #0  0x000000000058b89f in extract_unsigned_integer (addr=0x2 <error: Cannot access memory at address 0x2>, len=2, byte_order=BFD_ENDIAN_LITTLE)
     at /home/pedro/gdb/mygit/src/gdb/findvar.c:109
 #1  0x00000000005a187a in unpack_bits_as_long (field_type=0x16cff70, valaddr=0x0, bitpos=16, bitsize=12) at /home/pedro/gdb/mygit/src/gdb/value.c:3347
 #2  0x00000000005a1b9d in unpack_value_bitfield (dest_val=0x1b5d9d0, bitpos=16, bitsize=12, valaddr=0x0, embedded_offset=0, val=0x1b5d8d0)
     at /home/pedro/gdb/mygit/src/gdb/value.c:3441
 #3  0x00000000005a2a5f in value_fetch_lazy (val=0x1b5d9d0) at /home/pedro/gdb/mygit/src/gdb/value.c:3958
 #4  0x00000000005a10a7 in value_primitive_field (arg1=0x1b5d8d0, offset=0, fieldno=0, arg_type=0x16d04c0) at /home/pedro/gdb/mygit/src/gdb/value.c:3161
 #5  0x00000000005b01e5 in do_search_struct_field (name=0x1727c60 "bitfield", arg1=0x1b5d8d0, offset=0, type=0x16d04c0, looking_for_baseclass=0, result_ptr=0x7fffffffcaf8,
 [...]

unpack_value_bitfield is already optimized-out/unavailable -aware:

   (...) VALADDR points to the contents of VAL.  If the VAL's contents
   required to extract the bitfield from are unavailable/optimized
   out, DEST_VAL is correspondingly marked unavailable/optimized out.

however, it is not considering the case of the value having no
contents buffer at all, as can happen through
allocate_optimized_out_value.

gdb/ChangeLog:
2016-08-09  Pedro Alves  <palves@redhat.com>

	* value.c (unpack_value_bitfield): Skip unpacking if the parent
	has no contents buffer to begin with.

gdb/testsuite/ChangeLog:
2016-08-09  Pedro Alves  <palves@redhat.com>

	* gdb.dwarf2/bitfield-parent-optimized-out.exp: New file.