lo-regexp.cc: prevent cast of negative floating point value to size_t. * regexp.cc: simplify two expressions like "static_cast<size_t> (e - 1) + 1" to "static_cast<size_t> (e), where 'e' is a nonnegative index value of type double. This prevents a cast of -1.0 to the unsigned integral type "size_t" whenever 'e' is zero. A test case where this occurs (for the second change) is regexprep ('World', '^', 'Hello ', 'emptymatch').