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".

test/core-test.cc

@@ -103,7 +103,7 @@ template <typename T> struct mock_buffer : buffer<T> {
 TEST(BufferTest, Ctor) {
   {
     mock_buffer<int> buffer;
-    EXPECT_EQ(nullptr, &buffer[0]);
+    EXPECT_EQ(nullptr, buffer.data());
     EXPECT_EQ(static_cast<size_t>(0), buffer.size());
     EXPECT_EQ(static_cast<size_t>(0), buffer.capacity());
   }