Fix size check in dwarf2_evaluate_loc_desc_full

This Rust bug report:

https://github.com/rust-lang/rust/issues/41970

noted an error from gdb.  What is happening here (for me, the original
report had a different error) is that a pieced DWARF expression is not
writing to every byte in the resulting value.  GDB errors in this
case.  However, it seems to me that it is always valid to write fewer
bytes; the issue comes from writing too many -- that is, the test is
reversed.  The test was also checking the sub-object, but this also
seems incorrect, as it's expected for the expression to write the
entirety of the enclosing object.  So, this patch reverses the test
and applies it to the outer type, not the subobject type.

Regtested on the buildbot.

gdb/ChangeLog
2017-07-09  Tom Tromey  <tom@tromey.com>

	* dwarf2loc.c (dwarf2_evaluate_loc_desc_full): Reverse size
	check and apply to outer type.

gdb/testsuite/ChangeLog
2017-07-09  Tom Tromey  <tom@tromey.com>

	* gdb.dwarf2/shortpiece.exp: New file.