Return nil if accesing OOB section

Summary: Rare crashers that take `NSNotFound` from `sectionFor...` API and then immediately look up the object w/out checking for not found. Instead return nil.

Reviewed By: dshahidehpour

Differential Revision: D4142946

fbshipit-source-id: ca80f87729b5ee6699740de897d73b819d27d132

Source/Internal/IGListSectionMap.m

@@ -75,7 +75,12 @@
 }
 
 - (id)objectForSection:(NSUInteger)section {
-    return self.objects[section];
+    NSArray *objects = self.objects;
+    if (section < objects.count) {
+        return objects[section];
+    } else {
+        return nil;
+    }
 }
 
 - (NSUInteger)sectionForObject:(id)object {

Tests/IGListSectionMapTests.m

@@ -82,4 +82,12 @@
     XCTAssertEqual(counter, 2);
 }
 
+- (void)test_whenAccessingOOBSection_thatNilIsReturned {
+    NSArray *objects = @[@0, @1, @2];
+    NSArray *sectionControllers = @[@"a", @"b", @"c"];
+    IGListSectionMap *map = [[IGListSectionMap alloc] initWithMapTable:[NSMapTable strongToStrongObjectsMapTable]];
+    [map updateWithObjects:objects sectionControllers:sectionControllers];
+    XCTAssertNil([map objectForSection:4]);
+}
+
 @end