apps/cmp.c: fix bug not allowing to reset -csr and -serial option values

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21659)

(cherry picked from commit 374945a9aa)

apps/cmp.c

@@ -2512,7 +2512,7 @@ static int get_opts(int argc, char **argv)
             }
             break;
         case OPT_CSR:
-            opt_csr = opt_arg();
+            opt_csr = opt_str();
             break;
         case OPT_OUT_TRUSTED:
             opt_out_trusted = opt_str();