Segfault accessing Bayer Data

Hi there,
I'm new on this forum and I need your help.
So far I am trying to read Raw data, accessing the Bayer data. My code is working well except I have sometimes segfault on the same line: sometimes I read one pic, sometime I segfault on the same pic.

Here a piece of my code :

	libraw_data_t *raw = libraw_init(0);
	int i;
	int npixels;
	unsigned short *data=NULL;
	int ret = libraw_open_file(raw, name);
	ret = libraw_unpack(raw);
	int raw_width = raw->sizes.raw_width;
	int raw_height = raw->sizes.raw_height;
	npixels = raw_width * raw_height;	
	data = (WORD*) calloc(1, npixels * sizeof(WORD));
	if (!data) {
		return -1;
	WORD *buf = data;
	int offset = raw_width * raw->rawdata.sizes.top_margin + raw->rawdata.sizes.left_margin;	
	for (i = 0; i < npixels; i++) {
		buf[i] = raw->rawdata.raw_image[i + offset];

The line causing trouble is
buf[i] = raw->rawdata.raw_image[i + offset];

Program received signal SIGSEGV, Segmentation fault.
0x000000000043f20d in readraw_in_cfa (name=0x1056410 "/home/lock/Projet/Pictures/RAW/trails/IMG_3828.CR2", fit=0x666d60 <gfit>) at image_formats_libraries.c:790
790			buf[i] = raw->rawdata.raw_image[i + offset];

But as I said, sometimes it is wotking very well and I see no "pattern" in the segfault.
Did I make something wrong ?

Thanks for your help,