* Fix undefined in core-test
Fixes "reference binding to null pointer" in BufferTest.Ctor
buffer.operator[] attempts to return a reference to `buffer.ptr_[0]` when `ptr_`
in `mock_buffer<int> buffer` is null.
* Fix undefined in printf-test
Fixes "signed integer overflow" in PrintfTest.Length
This occurs in `TestLength<long long>("ll")`, since its minimum value minus one
does not fit in long long.
* Fix undefined in printf %0$
Printf counts arguments from 1.
Fixes "shift exponent -4 is negative" in PrintfTest.InvalidArgIndex.
`do_get` is called with index -1 when `basic_printf_context.arg` is called with
id 4294967295 when basic_printf_context::get_arg subtracts 1 from arg_index 0 in
the format string "%0$d".
b66bb6b71f