Fix crash in DependentGenericTypeResolver::resolveDependentMemberType

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

validation-test/compiler_crashers_fixed/28057-swift-modulefile-declcommenttableinfo-readdata.swift

@@ -0,0 +1,12 @@
+// 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
+
+class S<T{
+enum b:A
+func a{
+protocol B{
+func f<T where g=e
+func g:T.C