make sure we don't pass undefined file

fix #175

Author: WebFreak001

src/backend/mi2/mi2.ts

@@ -622,10 +622,12 @@ export class MI2 extends EventEmitter implements IBackend {
 			const func = MINode.valueOf(element, "@frame.func");
 			const filename = MINode.valueOf(element, "@frame.file");
 			let file: string = MINode.valueOf(element, "@frame.fullname");
-			if (this.isSSH)
-				file = posix.normalize(file);
-			else
-				file = nativePath.normalize(file);
+			if (file) {
+				if (this.isSSH)
+					file = posix.normalize(file);
+				else
+					file = nativePath.normalize(file);
+			}
 
 			let line = 0;
 			const lnstr = MINode.valueOf(element, "@frame.line");