Fix for crash for auto import completions with a rooted rootDirs entry (#47411)

* Add failing test case.

* Guard against undefined relative path.

Author: DanielRosenwasser

src/compiler/moduleSpecifiers.ts

@@ -854,8 +854,8 @@ namespace ts.moduleSpecifiers {
 
     function getPathRelativeToRootDirs(path: string, rootDirs: readonly string[], getCanonicalFileName: GetCanonicalFileName): string | undefined {
         return firstDefined(rootDirs, rootDir => {
-            const relativePath = getRelativePathIfInDirectory(path, rootDir, getCanonicalFileName)!; // TODO: GH#18217
-            return isPathRelativeToParent(relativePath) ? undefined : relativePath;
+            const relativePath = getRelativePathIfInDirectory(path, rootDir, getCanonicalFileName);
+            return relativePath !== undefined && isPathRelativeToParent(relativePath) ? undefined : relativePath;
         });
     }
 

tests/cases/fourslash/autoImportsWithRootDirsAndRootedPath01.ts

@@ -0,0 +1,24 @@
+/// <reference path="./fourslash.ts" />
+
+// @Filename: /dir/foo.ts
+//// export function foo() {}
+
+// @Filename: /dir/bar.ts
+//// /*$*/
+
+// @Filename: /dir/tsconfig.json
+////{
+////    "compilerOptions": {
+////        "module": "amd",
+////        "moduleResolution": "classic",
+////        "rootDirs": ["D:/"]
+////    }
+////}
+
+goTo.marker("$");
+verify.completions({
+    preferences: {
+        includeCompletionsForModuleExports: true,
+        allowIncompleteCompletions: true,
+    }
+});