Refactor c/isogram/length to exclude null char

Since we're no longer copying, we don't need to include that char in our
length. It just adds an extra iteration step.

c/isogram/src/isogram.c

@@ -4,7 +4,7 @@
 int length(const char string[]) {
   int i = 0;
   while (string[i] != '\0') { i++; }
-  return i+1; // Account for terminating null char.
+  return i;
 }
 
 char lowercase(char c) {