libfdt: Fix a possible "unchecked return value" warning

Apparently the unchecked return value of the first fdt_next_tag() call in
fdt_add_subnode_namelen() is tripping Coverity Scan in some circumstances,
although it appears not to for the scan on our project itself.

This fdt_next_tag() should always return FDT_BEGIN_NODE, since otherwise
the fdt_subnode_offset_namelen() above would have returned BADOFFSET or
BADSTRUCTURE.

Still, add a check to shut Coverity up, gated by a can_assume() to avoid
bloat in small builds.

Reported-by: Ryan Long <ryan.long@oarcorp.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_rw.c

@@ -349,7 +349,10 @@ int fdt_add_subnode_namelen(void *fdt, int parentoffset,
 		return offset;
 
 	/* Try to place the new node after the parent's properties */
-	fdt_next_tag(fdt, parentoffset, &nextoffset); /* skip the BEGIN_NODE */
+	tag = fdt_next_tag(fdt, parentoffset, &nextoffset);
+	/* the fdt_subnode_offset_namelen() should ensure this never hits */
+	if (!can_assume(LIBFDT_FLAWLESS) && (tag != FDT_BEGIN_NODE))
+		return -FDT_ERR_INTERNAL;
 	do {
 		offset = nextoffset;
 		tag = fdt_next_tag(fdt, offset, &nextoffset);