Fix undefined in core-test and printf-test

1. The change in include/fmt/core.h 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".

2. The change in test/core-test.cc 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.

2. The change in test/printf-test.cc 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.

include/fmt/core.h

@@ -1208,6 +1208,7 @@ template <typename Context> class basic_format_args {
 
   format_arg do_get(int index) const {
     format_arg arg;
+    if (index < 0) return arg;
     if (!is_packed()) {
       auto num_args = max_size();
       if (index < num_args) arg = args_[index];

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());
   }

test/printf-test.cc

@@ -337,7 +337,9 @@ template <typename T> void TestLength(const char* length_spec) {
   TestLength<T>(length_spec, -42);
   TestLength<T>(length_spec, min);
   TestLength<T>(length_spec, max);
-  TestLength<T>(length_spec, static_cast<long long>(min) - 1);
+  long long long_long_min = std::numeric_limits<long long>::min();
+  if (static_cast<long long>(min) > long_long_min)
+    TestLength<T>(length_spec, static_cast<long long>(min) - 1);
   unsigned long long long_long_max = max_value<long long>();
   if (static_cast<unsigned long long>(max) < long_long_max)
     TestLength<T>(length_spec, static_cast<long long>(max) + 1);