Some remaining malloc() calls -> safe_alloc()

Unimportant, but this way 100% of those is done.
This is step 4 of X of the memory function / caller changes.

src/debug.c

@@ -80,9 +80,9 @@ void	flag_add(char ch)
 	}
 	else
 	{
-		extraflags = malloc(2);
+		extraflags = safe_alloc(2);
 		extraflags[0] = ch;
-		extraflags[1] = 0;
+		extraflags[1] = '\0';
 	}
 }