Fix reading of undefined NativeType values in old Cecil
cil-strip crashed when processing this method: https://github.com/mono/corefx/blob/
f54e64b341a7b114c3502e197b46670cacb28639/src/Microsoft.CSharp/tests/DefaultParameterTests.cs#L88
It uses `MarshalAs((UnmanagedType)2000)`, i.e. an undefined value for the
native type. The old Cecil embedded in cil-strip tried to read more than one
byte which blows up in this case. Upstream Cecil just reads a single byte:
https://github.com/jbevain/cecil/blob/
1b79b96d29f1e8e9ba832191b1b42ea9cb750d20/Mono.Cecil/AssemblyReader.cs#L3700-L3703
Fixes https://github.com/mono/mono/issues/13639