AK: Use ByteBuffer::append for the StringBuilder class

Previously the StringBuilder class would use memcpy() to write
directly into the ByteBuffer's buffer. Instead we should use the
append() method which ensures we don't overrun the buffer.

AK/StringBuilder.cpp

@@ -26,12 +26,12 @@ inline void StringBuilder::will_append(size_t size)
     if (needed_capacity > inline_capacity)
         expanded_capacity *= 2;
     VERIFY(!expanded_capacity.has_overflow());
-    m_buffer.resize(expanded_capacity.value());
+    m_buffer.ensure_capacity(expanded_capacity.value());
 }
 
 StringBuilder::StringBuilder(size_t initial_capacity)
-    : m_buffer(decltype(m_buffer)::create_uninitialized(initial_capacity))
 {
+    m_buffer.ensure_capacity(initial_capacity);
 }
 
 void StringBuilder::append(const StringView& str)
@@ -39,7 +39,7 @@ void StringBuilder::append(const StringView& str)
     if (str.is_empty())
         return;
     will_append(str.length());
-    memcpy(data() + m_length, str.characters_without_null_termination(), str.length());
+    m_buffer.append(str.characters_without_null_termination(), str.length());
     m_length += str.length();
 }
 
@@ -51,7 +51,7 @@ void StringBuilder::append(const char* characters, size_t length)
 void StringBuilder::append(char ch)
 {
     will_append(1);
-    data()[m_length] = ch;
+    m_buffer.append(&ch, 1);
     m_length += 1;
 }