bench: fail early if user inputs invalid value for SECP256K1_BENCH_ITERS

In this change the get_iters function was updated to print an error
message and then return 0. In the functions that use get_iters they
print the help text and then EXIT_FAILURE

src/bench.h

@@ -150,7 +150,13 @@ static int have_invalid_args(int argc, char** argv, char** valid_args, size_t n)
 static int get_iters(int default_iters) {
     char* env = getenv("SECP256K1_BENCH_ITERS");
     if (env) {
-        return strtol(env, NULL, 0);
+        char* endptr;
+        long int iters = strtol(env, &endptr, 0);
+        if (*endptr != '\0' || iters <= 0) {
+            printf("Error: Value of SECP256K1_BENCH_ITERS is not a positive integer: %s\n\n", env);
+            return 0;
+        }
+        return iters;
     } else {
         return default_iters;
     }