If a listing is asked for, a "junk at end of line" error will be generated for every content line of a macro (other than the "macro" and "endm" lines themselves).

This appears to be as a result of the line not being consumed in the normal fashion by moving ptr, but by separate tokenisation, so a listing buffer is formed in the usual fashion, but then ptr still points to the final token(s), which will produce an error.

This case is already dealt with in the block that deals with "if", so a simple solution is to clear out ptr in the same fashion during macro definition, i.e. directly after

if (cmd == ENDM)
  define_macro = 0;

Add a new line:

ptr = "";