remove additional zero byte when writing pub der

Remove `- 1` for setting location of output buffer,
which added a leading zero which cause failure in ASN1 parsing.
Fixes #1257
diff --git a/programs/pkey/key_app_writer.c b/programs/pkey/key_app_writer.c
index 9d12077..b273e7d 100644
--- a/programs/pkey/key_app_writer.c
+++ b/programs/pkey/key_app_writer.c
@@ -128,7 +128,7 @@
             return( ret );
 
         len = ret;
-        c = output_buf + sizeof(output_buf) - len - 1;
+        c = output_buf + sizeof(output_buf) - len;
     }
 
     if( ( f = fopen( output_file, "w" ) ) == NULL )