Make plist_bool_val_is_true() return 0 instead of -1 if node is NULL or not a PLIST_BOOLEAN
diff --git a/src/plist.c b/src/plist.c
index 38212d8..27f90b1 100644
--- a/src/plist.c
+++ b/src/plist.c
@@ -1189,7 +1189,7 @@
 PLIST_API int plist_bool_val_is_true(plist_t boolnode)
 {
     if (!PLIST_IS_BOOLEAN(boolnode)) {
-        return -1;
+        return 0;
     }
     uint8_t bv = 0;
     plist_get_bool_val(boolnode, &bv);