image-fit: fit_check_format check for valid FDT

fit_check_format() must check that the buffer contains a flattened device
tree before calling any device tree library functions.

Failure to do may cause segmentation faults.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/common/image-fit.c b/common/image-fit.c
index 6a8787c..21c44bd 100644
--- a/common/image-fit.c
+++ b/common/image-fit.c
@@ -1553,6 +1553,12 @@
  */
 int fit_check_format(const void *fit)
 {
+	/* A FIT image must be a valid FDT */
+	if (fdt_check_header(fit)) {
+		debug("Wrong FIT format: not a flattened device tree\n");
+		return 0;
+	}
+
 	/* mandatory / node 'description' property */
 	if (fdt_getprop(fit, 0, FIT_DESC_PROP, NULL) == NULL) {
 		debug("Wrong FIT format: no description\n");