Fix bug in quirk_rust_enum

Tom de Vries pointed out that some Rust tests were failing after the
variant part rewrite.  He sent an executable, which helped track down
this bug.

quirk_rust_enum was passing 1 to alloc_rust_variant in one case.
However, a comment earlier says:

      /* We don't need a range entry for the discriminant, but we do
	 need one for every other field, as there is no default
	 variant.  */

In this case, we must pass -1 for this parameter.  That is what this
patch implements.

gdb/ChangeLog
2020-06-30  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (quirk_rust_enum): Correctly call
	alloc_rust_variant for default-less enum.