Correct base calculation in anyref allocator

This `base` value is the raw value coming out of the first call to
`table.grow`. Throwing in the addition of `JSIDX_RESERVED` just wasn't
right!

src/anyref.rs

@@ -39,7 +39,7 @@ impl Slab {
                     internal_error("table grow failure")
                 }
                 if self.base == 0 {
-                    self.base = r as usize + (super::JSIDX_RESERVED as usize);
+                    self.base = r as usize;
                 } else if self.base + self.data.len() != r as usize {
                     internal_error("someone else allocated table entires?")
                 }