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.c

@@ -177,8 +177,6 @@ int main(int argc, char** argv) {
     bench_data data;
 
     int d = argc == 1;
-    int default_iters = 20000;
-    int iters = get_iters(default_iters);
 
     /* Check for invalid user arguments */
     char* valid_args[] = {"ecdsa", "verify", "ecdsa_verify", "sign", "ecdsa_sign", "ecdh", "recover",
@@ -188,6 +186,13 @@ int main(int argc, char** argv) {
     size_t valid_args_size = sizeof(valid_args)/sizeof(valid_args[0]);
     int invalid_args = have_invalid_args(argc, argv, valid_args, valid_args_size);
 
+    int default_iters = 20000;
+    int iters = get_iters(default_iters);
+    if (iters == 0) {
+        help(default_iters);
+        return EXIT_FAILURE;
+    }
+
     if (argc > 1) {
         if (have_flag(argc, argv, "-h")
            || have_flag(argc, argv, "--help")

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;
     }

src/bench_ecmult.c

@@ -313,6 +313,10 @@ int main(int argc, char **argv) {
 
     int default_iters = 10000;
     int iters = get_iters(default_iters);
+    if (iters == 0) {
+        help(argv, default_iters);
+        return EXIT_FAILURE;
+    }
 
     data.ecmult_multi = secp256k1_ecmult_multi_var;
 

src/bench_internal.c

@@ -385,9 +385,13 @@ static void bench_context(void* arg, int iters) {
 
 int main(int argc, char **argv) {
     bench_inv data;
+    int d = argc == 1; /* default */
     int default_iters = 20000;
     int iters = get_iters(default_iters);
-    int d = argc == 1; /* default */
+    if (iters == 0) {
+        help(default_iters);
+        return EXIT_FAILURE;
+    }
 
     if (argc > 1) {
         if (have_flag(argc, argv, "-h")