kunit: string-stream: Don't create a fragment for empty strings

If the result of the formatted string is an empty string just return
instead of creating an empty fragment.
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

lib/kunit/string-stream.c

@@ -50,11 +50,17 @@ int string_stream_vadd(struct string_stream *stream,
 	/* Make a copy because `vsnprintf` could change it */
 	va_copy(args_for_counting, args);
 
-	/* Need space for null byte. */
-	len = vsnprintf(NULL, 0, fmt, args_for_counting) + 1;
+	/* Evaluate length of formatted string */
+	len = vsnprintf(NULL, 0, fmt, args_for_counting);
 
 	va_end(args_for_counting);
 
+	if (len == 0)
+		return 0;
+
+	/* Need space for null byte. */
+	len++;
+
 	frag_container = alloc_string_stream_fragment(stream->test,
 						      len,
 						      stream->gfp);