Add an example of undefined behaviour

undefined-behaviour.c

@@ -0,0 +1,16 @@
+#include <stdio.h>
+
+int main(void) {
+  int a = 5;
+  int b = a + 7;
+  int c = a - b + 7;
+
+  // c is zero now!
+  printf("c = %i\n", c);
+
+  // Depending on e.g. the optimization level, this either throws a
+  // floating point exception or does other things like returning 10.
+  if (3/c > 4)
+    return 5;
+  return 10;
+}