demo/allegro5: minor string-related tweaks

[lockie]

Hello! This tiny PR consists of a few string handling tweaks in allegro5 backend:

• local variable strcpy renamed to str, as per SEI CERT DCL01-C.
• malloc and manual null-terminator addition replaced with calloc.
• added missing NULL from allocator check.
• strcpy replaced with strncpy for better security.

[RobLoach]

Worked locally. Thanks!