Deal with the consequences of constifying getters

Reviewed-by: Stephen Henson <steve@openssl.org>
Reviewed-by: Emilia Käsper <emilia@openssl.org>
diff --git a/apps/dsaparam.c b/apps/dsaparam.c
index cd2ca4c..4042660 100644
--- a/apps/dsaparam.c
+++ b/apps/dsaparam.c
@@ -215,7 +215,7 @@
     }
 
     if (C) {
-        BIGNUM *p = NULL, *q = NULL, *g = NULL;
+        const BIGNUM *p = NULL, *q = NULL, *g = NULL;
         unsigned char *data;
         int len, bits_p;