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>

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, "\"");