libfoundation: MCStringCreateMutable(): Set length of string to zero.

When `MCStringCreateMutable()` is called with a non-zero
`p_initial_capacity`, `__MCStringExpandAt()` provides a buffer of the
requested length for the `__MCString`.  However, it also sets the
char_count of the `__MCString` to the same length.  This can cause the
resulting `__MCString` to contain arbitrary data.