libwebp MODE_RGB decoding doesn't handle Alpha value

Makefile

PROG=		webp-test-MODE_RGB
SRC=		webp-test-MODE_RGB.c

#PREFIX=		/usr/pkg
PREFIX=		/usr
CFLAGS=		-I${PREFIX}/include
LDFLAGS=	-L${PREFIX}/lib -Wl,-R${PREFIX}/lib -lwebp 
WEBPIMG=	3_webp_ll.webp

all:	${PROG}

${PROG}:	${SRC}
	cc ${SRC} -o ${PROG} ${CFLAGS} ${LDFLAGS}

test:	${PROG} ${WEBPIMG}
	./${PROG}

${WEBPIMG}:
	wget https://www.gstatic.com/webp/gallery3/${WEBPIMG}

clean:
	rm -f ${PROG}
	rm -f 3_webp_ll_RGB.webp

webp-test-MODE_RGB.c

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <inttypes.h>
#include <err.h>
#include <sys/stat.h>

#include <webp/types.h>
#include <webp/encode.h>
#include <webp/decode.h>

#define	IFNAME	"3_webp_ll.webp"
#define OFNAME	"3_webp_ll_RGB.webp"

int
main(int argc, char *argv[])
{
	FILE *ifp, *ofp;
	struct stat sb;
	int fd;
	int width, height, stride;
	size_t filesize, readsize, outsize, writesize;
	uint8_t *filebuf, *ipic, *opic;

	fd = open(IFNAME, O_RDONLY);
	if (fd == -1) {
		errx(EXIT_FAILURE, "failed to open input file");
	}

	if (fstat(fd, &sb) == -1) {
		errx(EXIT_FAILURE, "failed to stat input file");
	}
	filesize = sb.st_size;

	filebuf = calloc(filesize, 1);
	if (filebuf == NULL)
		errx(EXIT_FAILURE, "cannot allocate input buffer");

	ifp = fdopen(fd, "r");
	if (ifp == NULL) {
		errx(EXIT_FAILURE, "failed to open input stream");
	}
	readsize = fread(filebuf, 1, filesize, ifp);
	if (readsize != filesize) {
		errx(EXIT_FAILURE, "failed to read input file");
	}
	fclose(ifp);
	close(fd);

	/* decode transparent webp using MODE_RGB */
	ipic = WebPDecodeRGB(filebuf, filesize, &width, &height);
	free(filebuf);

	stride = width * 3;	/* Decoded ipic has RGB without A */
	outsize = WebPEncodeLosslessRGB(ipic, width, height, stride, &opic);
	WebPFree(ipic);

	ofp = fopen(OFNAME, "w");
	if (ofp == NULL) {
		errx(EXIT_FAILURE, "failed to open output file");
	}
  	writesize = fwrite(opic, 1, outsize, ofp);
	WebPFree(opic);
	if (writesize != outsize) {
		errx(EXIT_FAILURE, "failed to write output file");
	}
	fclose(ofp);

	exit(EXIT_SUCCESS);
}