Use basic_string::clear() instead of basic_string::assign(0, 0).

The latter is an ambiguous call and makes the build fail with libc++:

  /s/stepcode/src/cllazyfile/sectionReader.cc:86:9: error: call to member
    function 'assign' is ambiguous
      str.assign( 0, 0 ); //clear() frees the memory
      ~~~~^~~~~~
  /usr/include/c++/v1/string:1354:19: note: candidate function
      basic_string& assign(const value_type* __s, size_type __n);
                    ^
  /usr/include/c++/v1/string:1356:19: note: candidate function
      basic_string& assign(size_type __n, value_type __c);
                    ^
  /usr/include/c++/v1/string:1360:14: note: candidate template ignored:
    disabled by 'enable_if' [with _InputIterator = int]
               __is_input_iterator  <_InputIterator>::value &&
               ^
  /usr/include/c++/v1/string:1368:13: note: candidate template ignored:
    disabled by 'enable_if' [with _ForwardIterator = int]
              __is_forward_iterator<_ForwardIterator>::value,
              ^
  /usr/include/c++/v1/string:1347:19: note: candidate function not viable:
    requires single argument '__str', but 2 arguments were provided
      basic_string& assign(const basic_string& __str);
                    ^
  /usr/include/c++/v1/string:1353:19: note: candidate function not viable:
    requires 3 arguments, but 2 were provided
      basic_string& assign(const basic_string& __str, size_type __pos,
    size_type __n);
                    ^
  /usr/include/c++/v1/string:1355:19: note: candidate function not viable:
    requires single argument '__s', but 2 arguments were provided
      basic_string& assign(const value_type* __s);
                    ^

Instead, just call clear(), which should do the same thing and, contrary to
what the comment stated, does not clear the allocated memory.
Merge pull request #269 from rakuco/string-clear-not-assign

Use basic_string::clear() instead of basic_string::assign(0, 0).