Fix potential integer overflow in line_atlas

[springmeyer]

Spotted thanks to clang++ -fsanitize=undefined:

line_atlas.cpp:115: runtime error: value 249 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 249 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 250 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 251 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 252 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 253 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 254 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 255 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 255 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 255 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 255 is outside the range of representable values of type 'char'
line_atlas.cpp:115: runtime error: value 255 is outside the range of representable values of type 'char'

[kkaefer]

We should replace this with std::unique_ptr managed code.

[kkaefer]

Please review.

[incanus]

Similar to #1485, does this have any practical implications for b1 or is it just housekeeping?