fix some crashes

colcat.c

@@ -73,7 +73,7 @@ int display(FILE *file) {
     int end;
     int counter = 0;
     if (frequency) 
-        end = num_colours / frequency - 1;
+        end = (num_colours / frequency);
     else {
         end = 0;
         counter = num_colours; // so that print starts at reverse=true
@@ -82,7 +82,7 @@ int display(FILE *file) {
     bool vert_reverse = false;
     while (c != EOF) {
         if (c == '\n') {
-            if (frequency) {
+            if (frequency && end) {
                 if (counter % (end) == 0) {
                     vert_reverse = !vert_reverse;
                 }