Minor adjustment to work on Windows

According to at least one report, with some compilers on Windows
(no idea what language / mode), the following line:

  const glyphy_rgba_t *needle = &tex_data[offset];

generates a runtime "vector subscript out of range" if offset is
equal to the length of the vector, even though getting a pointer
to the end of an array is a valid operation in C.  Anyway, work
around it.