Clang's ubsan (-fsanitize=undefined) reports:
runtime error: negation of -9223372036854775808 cannot be represented in
type 'Json::Value::LargestInt' (aka 'long'); cast to an unsigned type to
negate this value to itself
Follow its advice and update the code to remove the explicit negation.
The scanbuild analysis cannot see that decodeUnicodeEscapeSequence
either initializes the `unicode` argument or returns `false` such
that the code following it runs only if `unicode` is initialized.
Add an explicit initialization to pacify it.
When parsing digits we know our `c - '0'` expression results in a
non-negative value due to preceding conditions. Simply cast the result
to UInt. This fixes compilation on SolarisStudio 12.4.
