staging: fbtft: Fix potential null dereference

This error is found by smatch tool.
Signed-off-by: NMadhusudhanan Ravindran <mravindr@visteon.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fbtft/fb_agm1264k-fl.c

@@ -282,6 +282,9 @@ static int write_vmem(struct fbtft_par *par, size_t offset, size_t len)
 	signed short *convert_buf = kmalloc(par->info->var.xres *
 		par->info->var.yres * sizeof(signed short), GFP_NOIO);
 
+	if (!convert_buf)
+		return -ENOMEM;
+
 	fbtft_par_dbg(DEBUG_WRITE_VMEM, par, "%s()\n", __func__);
 
 	/* converting to grayscale16 */