Avoid crash for some special XPath queries by explicitly checking whether result nodeSet is empty or not

Summary:
Based on facebookarchive#403
Closes facebookarchive#404

Differential Revision: D4298605

Pulled By: marekcirkos

fbshipit-source-id: 44e9d91a26ceac42dc7f20e3ac0188f8013c0771

Author: Mykola Mokhnach

WebDriverAgentLib/Utilities/FBXPath.m

@@ -89,6 +89,9 @@ + (nullable NSString *)xmlStringWithSnapshot:(XCElementSnapshot *)root
 
 + (NSArray *)collectMatchingSnapshots:(xmlNodeSetPtr)nodeSet elementStore:(NSMutableDictionary *)elementStore
 {
+  if (xmlXPathNodeSetIsEmpty(nodeSet)) {
+    return @[];
+  }
   NSMutableArray *matchingSnapshots = [NSMutableArray array];
   const xmlChar *indexPathKeyName = [FBXPath xmlCharPtrForInput:[kXMLIndexPathKey cStringUsingEncoding:NSUTF8StringEncoding]];
   for (NSInteger i = 0; i < nodeSet->nodeNr; i++) {

WebDriverAgentTests/IntegrationTests/XCUIElementFBFindTests.m

@@ -186,4 +186,10 @@ - (void)testDescendantsWithPropertyPartial
   XCTAssertEqualObjects(matchingSnapshots.lastObject.label, @"Alerts");
 }
 
+- (void)testSpecialXPathQuery
+{
+  NSArray<XCUIElement *> *matchingSnapshots = [self.testedView fb_descendantsMatchingXPathQuery:@".//XCUIElementTypeButton[@label=\"Alerts\"]" shouldReturnAfterFirstMatch:NO];
+  XCTAssertEqual(matchingSnapshots.count, 0);
+}
+
 @end