Explicitly add terminating null character.

test/core/end2end/tests/payload.c

@@ -102,9 +102,10 @@ static gpr_slice generate_random_slice() {
   size_t i;
   static const char chars[] = "abcdefghijklmnopqrstuvwxyz1234567890";
   char output[1024 * 1024]; /* 1 MB */
-  for (i = 0; i < 1024 * 1024; ++i) {
+  for (i = 0; i < 1024 * 1024 - 1; ++i) {
     output[i] = chars[rand() % (int)(sizeof(chars) - 1)];
   }
+  output[1024 * 1024 - 1] = '\0';
   return gpr_slice_from_copied_string(output);
 }