libclang_rt_prebuilt_library_shared mixed builds

Also fix nil deref in setting $LOCAL_SOONG_UNSTRIPPED_BINARY

Bug: 201802518
Test: request_type_test.go
Test: prebuilt_test.go:TestPrebuiltLibrarySharedWithBazel
Test: mixed_{libc,droid}.sh
Change-Id: I22afb56c4b42d3412c2b2e1f079f1bcf8f3129a7
diff --git a/cc/androidmk.go b/cc/androidmk.go
index 800b58f..f6fe6f7 100644
--- a/cc/androidmk.go
+++ b/cc/androidmk.go
@@ -241,7 +241,7 @@
 		entries.Class = "SHARED_LIBRARIES"
 		entries.ExtraEntries = append(entries.ExtraEntries, func(_ android.AndroidMkExtraEntriesContext, entries *android.AndroidMkEntries) {
 			entries.SetString("LOCAL_SOONG_TOC", library.toc().String())
-			if !library.buildStubs() {
+			if !library.buildStubs() && library.unstrippedOutputFile != nil {
 				entries.SetString("LOCAL_SOONG_UNSTRIPPED_BINARY", library.unstrippedOutputFile.String())
 			}
 			if len(library.Properties.Overrides) > 0 {