AK: Fix unsigned integer underflow in DuplexMemoryStream::write.

Author: https://github.com/asynts
Commit: https://github.com/SerenityOS/serenity/commit/ce15c9a04cd
Pull-request: https://github.com/SerenityOS/serenity/pull/4371

AK/Forward.h

@@ -133,6 +133,15 @@ class WeakPtr;
 template<typename T, size_t inline_capacity = 0>
 class Vector;
 
+template<typename... Parameters>
+void dbgln(const char* fmtstr, const Parameters&...);
+
+template<typename... Parameters>
+void warnln(const char* fmtstr, const Parameters&...);
+
+template<typename... Parameters>
+void outln(const char* fmtstr, const Parameters&...);
+
 }
 
 using AK::Array;

AK/MemoryStream.h

@@ -289,7 +289,7 @@ public:
             if ((m_write_offset + nwritten) % chunk_size == 0)
                 m_chunks.append(ByteBuffer::create_uninitialized(chunk_size));
 
-            nwritten += bytes.copy_trimmed_to(m_chunks.last().bytes().slice(m_write_offset % chunk_size));
+            nwritten += bytes.slice(nwritten).copy_trimmed_to(m_chunks.last().bytes().slice(m_write_offset % chunk_size));
         }
 
         m_write_offset += nwritten;

AK/Tests/TestMemoryStream.cpp

@@ -208,4 +208,12 @@ TEST_CASE(offset_of_out_of_bounds)
     EXPECT(!stream.offset_of(target).has_value());
 }
 
+TEST_CASE(unsigned_integer_underflow_regression)
+{
+    Array<u8, DuplexMemoryStream::chunk_size + 1> buffer;
+
+    DuplexMemoryStream stream;
+    stream << buffer;
+}
+
 TEST_MAIN(MemoryStream)