Prevent arithmetic overflow parsing -0.
Prevent arithmetic overflow parsing -0.

Although negating a zero is inconsequential, the JSON specification does
allow it. Although the 2's complement of zero is zero, calculating it
causes an arithmetic overflow. Instead of panicking, check for a zero
value input and just return zero.

rust-lang-deprecated/rustc-serialize,rust-lang/rustc-serialize,rust-lang-nursery/rustc-serialize