client.c: remove trailing newline from output for '-o -'

The CLI option '-o file' redirects the received payload into the
given file, where '-' denotes stdout. As the function close_output()
used to append a newline to the end of the output, certain tools
such as cbor2pretty.rb have failed, complaining about the trailing
character.

This change now writes the newline if only if option '-o' was not
specified.

examples/client.c

@@ -129,8 +129,8 @@ static void
 close_output(void) {
   if (file) {
 
-    /* add a newline before closing in case were writing to stdout */
-    if (!output_file.s || (output_file.length && output_file.s[0] == '-'))
+    /* add a newline before closing if no option '-o' was specified */
+    if (!output_file.s)
       fwrite("\n", 1, 1, file);
 
     fflush(file);