Buffer: fix assertion

By a mistake a debug assertion was placed in a bit wrong place.

Move it to the right position.

Add a test.

Part of #28

Author: alyapunov

src/Buffer/Buffer.hpp

@@ -787,17 +787,17 @@ Buffer<N, allocator>::dropBack(size_t size)
 
 	/* Do not delete the block if it is empty after drop. */
 	while (TNT_UNLIKELY(size > left_in_block)) {
-		assert(!m_blocks.isEmpty());
-		delBlock(block);
-		block = &m_blocks.last();
-
 		/*
 		 * Make sure there's no iterators pointing to the block
 		 * to be dropped.
 		 */
 		assert(m_iterators.isEmpty() ||
 		       m_iterators.last().getBlock() != block);
 
+		assert(!m_blocks.isEmpty());
+		delBlock(block);
+		block = &m_blocks.last();
+
 		m_end = block->end();
 		size -= left_in_block;
 		left_in_block = Block::DATA_SIZE;

test/BufferUnitTest.cpp

@@ -214,6 +214,19 @@ buffer_basic()
 		buf2.dropBack(i);
 		fail_unless(*buf2.begin() == c);
 	}
+
+	// Check when there a heavy iterators.
+	for (size_t i = 0; i < N; i++) {
+		tnt::Buffer<N> buf2;
+		if (i != 0)
+			buf2.write({i});
+		itr = buf2.end();
+		for (size_t j = 1; j < N; j++) {
+			buf2.write({j});
+			buf2.dropBack(j);
+		}
+		fail_unless(buf2.end() == itr);
+	}
 }
 
 /**