Make patch.c fully `-Wconversion` compliant.
Make patch.c fully `-Wconversion` compliant.

This silences the last warning on Travis when compiling with MSVC.

sourcefrog/librsync,dbaarda/librsync,dbaarda/librsync,sourcefrog/librsync,librsync/librsync,dbaarda/librsync,sourcefrog/librsync,sourcefrog/librsync,librsync/librsync,librsync/librsync,dbaarda/librsync,librsync/librsync