Fix undefined in core-test and printf-test (#1345)

* 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".

include/fmt/printf.h

@@ -469,6 +469,8 @@ OutputIt basic_printf_context<OutputIt, Char>::format() {
 
     // Parse argument index, flags and width.
     unsigned arg_index = parse_header(it, end, specs);
+    if (arg_index == 0)
+      on_error("argument index 0 is out of range");
 
     // Parse precision.
     if (it != end && *it == '.') {