Fix clang warning; This is copying pointers, it should be safe.

In file included from String.cpp:25:
In file included from ./String.h:27:
./ReferenceCountedArray.h:90:17: warning: destination for this 'memmove' call is a pointer to dynamic class 'bd::String'; vtable pointer will be overwritten [-Wdynamic-class-memaccess]
        memmove(buf, buf + offset, sublen);
        ~~~~~~~ ^
./ReferenceCountedArray.h:508:12: note: in instantiation of member function 'bd::ArrayRef<bd::String, std::allocator<bd::String> >::Reserve' requested here
      Ref->Reserve(newSize, offset, sublen);
           ^
String.cpp:147:25: note: in instantiation of member function 'bd::ReferenceCountedArray<bd::String, std::allocator<bd::String> >::Reserve' requested here
  if (!length()) return Array<String>();
                        ^
./ReferenceCountedArray.h:90:17: note: explicitly cast the pointer to silence this warning
        memmove(buf, buf + offset, sublen);
                ^
                (void*)
1 warning generated.