gdb: fix off-by-one error in quirk_rust_enum

gdb: fix off-by-one error in quirk_rust_enum

Found by inspection, so I don't have a test for it (I don't think it
would be easy to have this bug cause a failure reliably).

We allocate space for N fields into `new_fields`, then memcpy N fields
at `new_fields + 1`.  This overflows the allocated buffer by one field.

Fix it by allocating `N + 1` fields.

gdb/ChangeLog:

	* dwarf2/read.c (quirk_rust_enum): Allocate enough fields.