libselinux: label_file::init(): do not pass NULL to strdup

If any of the build flags `BUILD_HOST` or `ANDROID` is set and the
caller did not pass an option of type `SELABEL_OPT_PATH`, the variable
`path` might be not set.
Add a check to avoid calling `strdup()` with a NULL pointer.

Found by cppcheck.

src/label_file.c:759:26: warning: Possible null pointer dereference: path [nullPointer]
 rec->spec_file = strdup(path);
                         ^
src/label_file.c:713:21: note: Assignment 'path=NULL', assigned value is 0
 const char *path = NULL;
                    ^
src/label_file.c:759:26: note: Null pointer dereference
 rec->spec_file = strdup(path);
                         ^

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

libselinux/src/label_file.c

@@ -756,6 +756,10 @@ static int init(struct selabel_handle *rec, const struct selinux_opt *opts,
 	}
 
 #endif
+
+	if (!path)
+		goto finish;
+
 	rec->spec_file = strdup(path);
 
 	/*