avoid fixed-size buffers in index exception code * lo-array-gripes.h, lo-array-gripes.cc (index_exception::expression): Use std::ostringstream instead of fixed size buffer. (gripe_invalid_index): Likewise. (out_of_range::details): Likewise. (index_exception::index_exception): Set default dim value to -1. * ov-complex.h, ov-complex.cc (gripe_complex_index): Now static. Pass arg by const reference instead of value. Use std::ostringstream instead of fixed size buffer. (octave_complex::index_vector): Move definition out of header file.

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