Reset the towstring() char pointer before converting
Reset the towstring() char pointer before converting

In mbsrtowcs(), when dst is a null pointer then src must be left
unchanged (ISO/IEC 9899:1999 7.24.6.4.1-3). However, Mingw-w64 advances
src when dst is a null pointer, which is a bug. As a workaround, reset
src back to the beginning of the input. This fixes #10.

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