Fix dts output with a REF_PATH marker

Commit 8c59a97ce0 ("Fix missing labels when emitting dts format")
fixed label output, but broke output when there is a REF_PATH marker.

The problem is a REF_PATH marker causes a zero length string to be
emitted. The write_propval_string() function requires a length of at
least 1 (including the terminating '\0'), but that was not being
checked.

For the integer output, a length of 0 is valid as it is possible to have
labels inside the starting '<':

int-prop = < start: 0x1234>;

REF_PHANDLE is another marker that we don't explicitly handle, but it
doesn't cause a problem as it is fundamentally just an int.

Fixes: 8c59a97ce0 ("Fix missing labels when emitting dts format")
Reported-by: Kumar Gala <kumar.gala@linaro.org>
Cc: Grant Likely <grant.likely@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/path-references.dts

@@ -16,7 +16,7 @@
 	foobar {
 		n3: baz {
 			ref = &{/foo/baz};
-			lref = &n4;
+			lref = start: &n4 end:;
 		};
 	};
 	foo {

tests/run_tests.sh

@@ -572,6 +572,12 @@ dtc_tests () {
         run_dtc_test -I dts -O dts $tree.test.dts
         run_wrap_test cmp $tree $tree.test.dts
     done
+    for tree in path-references; do
+        run_dtc_test -I dts -O dtb -o $tree.test.dtb $tree.dts
+        run_dtc_test -I dts -O dts -o $tree.test.dts $tree.dts
+        run_dtc_test -I dts -O dtb -o $tree.test.dts.test.dtb $tree.test.dts
+        run_test dtbs_equal_ordered $tree.test.dtb $tree.test.dts.test.dtb
+    done
 
     # Check -Oyaml output
     if pkg-config --exists yaml-0.1; then

treesource.c

@@ -64,6 +64,10 @@ static bool isstring(char c)
 static void write_propval_string(FILE *f, const char *s, size_t len)
 {
 	const char *end = s + len - 1;
+
+	if (!len)
+		return;
+
 	assert(*end == '\0');
 
 	fprintf(f, "\"");