openssl: Use dynamically allocated array to determine EC curves

This avoids the use of a variable length array, which should probably
be avoided in general due to potential performance, portability and
security issues (not in this particular case, though).

Closes strongswan/strongswan#1095

src/libstrongswan/plugins/openssl/openssl_plugin.c

@@ -345,13 +345,15 @@ static bool ecdh_group_supported(EC_builtin_curve *curves, size_t num_curves,
 static void add_ecdh_features(plugin_feature_t *features,
 							  plugin_feature_t *to_add, int count, int *pos)
 {
+	EC_builtin_curve *curves;
 	size_t num_curves;
 	int i;
 
 	num_curves = EC_get_builtin_curves(NULL, 0);
 
-	EC_builtin_curve curves[num_curves];
-
+	if (num_curves)
+	{
+		curves = calloc(num_curves, sizeof(EC_builtin_curve));
 		num_curves = EC_get_builtin_curves(curves, num_curves);
 
 		for (i = 0; i < count; i++)
@@ -362,6 +364,8 @@ static void add_ecdh_features(plugin_feature_t *features,
 				features[(*pos)++] = to_add[i];
 			}
 		}
+		free(curves);
+	}
 }
 #endif /* OPENSSL_NO_ECDH */