tools: imx8image: fix coverity CID 184234

Fix:
CID 184234:    (TAINTED_SCALAR)
Using tainted variable "header.num_images - 1" as an index into an array "header.img".

Reported-by: Coverity
Signed-off-by: NPeng Fan <peng.fan@nxp.com>

tools/imx8image.c

@@ -666,8 +666,10 @@ static int get_container_image_start_pos(image_t *image_stack, uint32_t align)
 			}
 
 			ret = fread(&header, sizeof(header), 1, fd);
-			if (ret != 1)
+			if (ret != 1) {
 				printf("Failure Read header %d\n", ret);
+				exit(EXIT_FAILURE);
+			}
 
 			fclose(fd);