Fix crash in DependentGenericTypeResolver::resolveDependentMemberType

An erroneous `baseTy` would cause resolveArchetype to return nullptr, resulting in a null dereference.

validation-test/compiler_crashers_fixed/27955-swift-declattribute-print.swift

@@ -0,0 +1,20 @@
+// RUN: not %target-swift-frontend %s -parse
+
+// Distributed under the terms of the MIT license
+// Test case submitted to project by https://github.com/practicalswift (practicalswift)
+// Test case found by fuzzing
+
+func b{
+{
+{
+{
+{
+{
+}}}}}
+struct B<T{class D{
+struct a{
+class d
+func u{
+{}}struct d
+}}
+enum S<T: T.b