Avoid potential memory leak in code generated by 'openssl dhparam -C'.

apps/dhparam.c

@@ -464,7 +464,7 @@ bad:
 		printf("\tdh->g=BN_bin2bn(dh%d_g,sizeof(dh%d_g),NULL);\n",
 			bits,bits);
 		printf("\tif ((dh->p == NULL) || (dh->g == NULL))\n");
-		printf("\t\treturn(NULL);\n");
+		printf("\t\t{ DH_free(dh); return(NULL); }\n");
 		if (dh->length)
 			printf("\tdh->length = %d\n", dh->length);
 		printf("\treturn(dh);\n\t}\n");