Improve handling of malformed base64

A 2-byte chunk is not valid per-se, but I want to maintain the behaviour prior to 13cfa0c as this was not meant to change any semantics.

CLI/cli_test.cpp

@@ -363,6 +363,11 @@ static void unit_data()
 			assert(base64::decode("YQ==") == "a");
 			assert(base64::decode("SGVsbG8=") == "Hello");
 			assert(base64::decode("8J+YgA==") == "😀");
+
+			assert(base64::decode("YWFh") == "aaa");
+			assert(base64::decode("YWF") == "aa");
+			assert(base64::decode("YW") == "a");
+			assert(base64::decode("a") == "h");
 		});
 		test("urlDecode", []
 		{

soup/base64.cpp

@@ -167,7 +167,7 @@ NAMESPACE_SOUP
 		size_t out_len = size / 4 * 3;
 		if (auto remainder = (size % 4))
 		{
-			out_len += (remainder - 1);
+			out_len += (remainder - 1) + (remainder == 1);
 		}
 		return out_len;
 	}
@@ -243,7 +243,7 @@ NAMESPACE_SOUP
 
 		if (auto remainder = (size % 4))
 		{
-			auto extra = (remainder - 1);
+			auto extra = (remainder - 1) + (remainder == 1);
 
 			uint32_t a = i != size ? table[static_cast<uint8_t>(data[i++])] : 0;
 			uint32_t b = i != size ? table[static_cast<uint8_t>(data[i++])] : 0;