[AST] getParameterAt should check index before retrieving it

`getParameterAt` needs to be more defensive about the input
because caller cannot always perform all the checks upfront.

Resolves: rdar://102085039

Author: xedin

lib/AST/Decl.cpp

@@ -7815,7 +7815,7 @@ const ParamDecl *swift::getParameterAt(ConcreteDeclRef declRef,
 const ParamDecl *swift::getParameterAt(const ValueDecl *source,
                                        unsigned index) {
   if (auto *params = getParameterList(const_cast<ValueDecl *>(source))) {
-    return params->get(index);
+    return index < params->size() ? params->get(index) : nullptr;
   }
   return nullptr;
 }

validation-test/Sema/type_checker_crashers_fixed/rdar102085039.swift

@@ -0,0 +1,40 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol ResultType {
+  associatedtype E
+  associatedtype F
+
+  func failure() -> F?
+}
+
+extension Result: ResultType {
+  typealias E = Success
+  typealias F = Failure
+
+  func failure() -> F? { fatalError() }
+}
+
+protocol ObservableType {
+    associatedtype Element
+}
+
+class Observable<Element> : ObservableType {
+  func flatMap<Source: ObservableType>(_ selector: @escaping (Element) throws -> Source) -> Observable<Source.Element> {
+    fatalError()
+  }
+
+  static func just(_ element: Element) -> Observable<Element> {
+    fatalError()
+  }
+}
+
+extension Observable where Element: ResultType {
+  func mapGenericError() -> Observable<Result<Element.E, Error>> {
+    flatMap { result -> Observable<Result<Element.E, Error>> in
+      if let error = result.failure() as? Error {
+        return .just(Result.failure(error)) // Ok
+      }
+      fatalError()
+    }
+  }
+}