Fix crash in RCTRedBox::formatFrameSource:

Summary:
Motivation: Similar to facebook#13242 - the application will crash with `[NSNull lastPathComponent]: unrecognized selector sent to instance` if a stack frame with no filename makes it to RCTRedBox.
Closes facebook#13360

Differential Revision: D4848980

Pulled By: javache

fbshipit-source-id: dc506f2ba39c2d4d68982b56a6e765a19dc048b0

React/Modules/RCTRedBox.m

@@ -185,8 +185,9 @@ - (void)copyStack
 
 - (NSString *)formatFrameSource:(RCTJSStackFrame *)stackFrame
 {
+  NSString *fileName = RCTNilIfNull(stackFrame.file) ? [stackFrame.file lastPathComponent] : @"<unknown file>";
   NSString *lineInfo = [NSString stringWithFormat:@"%@:%zd",
-                        [stackFrame.file lastPathComponent],
+                        fileName,
                         stackFrame.lineNumber];
 
   if (stackFrame.column != 0) {