StringBuilder: Reset the internal builder length after building.

This puts the StringBuilder back into a pristine state, allowing you
to use it to build more strings after you've built one.

AK/StringBuilder.cpp

@@ -61,6 +61,7 @@ void StringBuilder::appendf(const char* fmt, ...)
 ByteBuffer StringBuilder::to_byte_buffer()
 {
     m_buffer.trim(m_length);
+    m_length = 0;
     return move(m_buffer);
 }
 
@@ -68,6 +69,7 @@ String StringBuilder::to_string()
 {
     auto string = String((const char*)m_buffer.pointer(), m_length);
     m_buffer.clear();
+    m_length = 0;
     return string;
 }