Fix error span for invalid slice indexes

The span now properly points at the token that was invalid, rather than
the start of the slice.

Also fix the span for `()[1]` and `()[d]`, which were previously
reporting no source location at all.

tests/expansion.err

@@ -16,3 +16,15 @@ fish: show "$foo[2 1]"
 Array index out of bounds
 fish: show $foo[2 1]
                 ^
+Invalid index value
+fish: echo "$foo[d]"
+                 ^
+Invalid index value
+fish: echo $foo[d]
+                ^
+Array index out of bounds
+fish: echo ()[1]
+              ^
+Invalid index value
+fish: echo ()[d]
+              ^