nalPrefix.readUnsignedIntToInt() will throw if the unsigned output doesn't fit
in an int. Before this change, in the error case we'd retry reading from after
the NAL prefix position. Throw a ParserException so that reading an obviously
invalid NAL prefix will be treated as a fatal exception by the default load
error handling policy.
PiperOrigin-RevId: 236839690
febb8c589e