Fix DecodeBase64 to reject truncated input

If the input doesn't have the proper number of encoding characters
(a multiple of 4), return an empty result.

Co-Authored-By: Sean Curtis <sean.curtis@tri.global>

test/binary_test.cpp

@@ -12,3 +12,9 @@ TEST(BinaryTest, DecodingNoCrashOnNegative) {
   const std::vector<unsigned char> &result = YAML::DecodeBase64(input);
   EXPECT_TRUE(result.empty());
 }
+
+TEST(BinaryTest, DecodingTooShort) {
+  std::string input{90, 71, 86, 104, 90, 71, 74, 108, 90, 87, 89};
+  const std::vector<unsigned char> &result = YAML::DecodeBase64(input);
+  EXPECT_TRUE(result.empty());
+}