return NULL string when spiffy decode errors when getting a string
diff --git a/src/qcbor_decode.c b/src/qcbor_decode.c
index 57b9bce..5859957 100644
--- a/src/qcbor_decode.c
+++ b/src/qcbor_decode.c
@@ -3752,6 +3752,8 @@
 
    if(pMe->uLastError == QCBOR_SUCCESS) {
       *pBstr = Item.val.string;
+   } else {
+      *pBstr = NULLUsefulBufC;
    }
 }