check if count is negative as well

Co-authored-by: Tim Ruffing <crypto@timruffing.de>

src/tests.c

@@ -5632,7 +5632,7 @@ int main(int argc, char **argv) {
             count = strtol(env, NULL, 0);
         }
     }
-    if (count == 0) {
+    if (count <= 0) {
         fputs("An iteration count of 0 is not allowed.\n", stderr);
         return EXIT_FAILURE;
     }