buffer: remove a pairs of asserts

There were 3 assert that assured that read and write data are of
non-zero size. Actually there were no harm in it. More important
is that in mpp code writing and reading of zero-size strings must
be a valid case.

So just remove asserts and add a test of ecoding/descoding of an
empty string.

Author: alyapunov

src/Buffer/Buffer.hpp

@@ -675,8 +675,6 @@ template <size_t N, class allocator>
 void
 Buffer<N, allocator>::write(WData data)
 {
-	assert(data.size != 0);
-
 	char *new_end = m_end + data.size;
 	if (TNT_LIKELY(isSameBlock(m_end, new_end))) {
 		// new_addr is still in block. just copy and advance.
@@ -1132,7 +1130,6 @@ template <bool LIGHT>
 void
 Buffer<N, allocator>::iterator_common<LIGHT>::write(WData data)
 {
-	assert(data.size > 0);
 	size_t left_in_block = N - (uintptr_t) m_position % N;
 	while (TNT_UNLIKELY(data.size >= left_in_block)) {
 		std::memcpy(m_position, data.data, left_in_block);
@@ -1238,7 +1235,6 @@ template <bool LIGHT>
 void
 Buffer<N, allocator>::iterator_common<LIGHT>::read(RData data)
 {
-	assert(data.size > 0);
 	/*
 	 * The same implementation as in ::set() method buf vice versa:
 	 * buffer and data sources are swapped.

test/EncDecTest.cpp

@@ -230,6 +230,7 @@ test_basic()
 	mpp::encode(buf, std::integral_constant<bool, false>{});
 	mpp::encode(buf, std::integral_constant<bool, true>{});
 	// Strings.
+	mpp::encode(buf, "");
 	mpp::encode(buf, "abc");
 	const char *bbb = "defg";
 	mpp::encode(buf, bbb);
@@ -322,7 +323,9 @@ test_basic()
 	fail_if(bt != true);
 
 	// Strings.
-	std::string a;
+	std::string a = "nothing";
+	fail_unless(mpp::decode(run, a));
+	fail_if(a != "");
 	char b_data[10];
 	size_t b_size = 0;
 	auto b = tnt::make_ref_vector(b_data, b_size);