must be valid. Oopses on bad direction characters.
(pathtoxy, ac_encounter): Bad direction characters used to lead to bad
array subscript and potential disaster.
(pathrange, path): Stop on DIR_STOP as well as on bad direction
characters. This is just for consistency with other code; DIR_STOP
should occur only last in a path here.
(sail_nav_fleet, nav_ship): No change except for the oops.
effect. Replace calls by struct assignment where possible. Replace
clear buffer, copy string to buffer by strncpy(). Use assignment to
clear when that's clearer. Replace overlapping copy through bounce
buffer by memmove(). Replace rest by standard memset() and memcpy().
Also use sizeof() instead of literal array sizes for robustness, and
instead of symbolic array sizes for clarity.
