mi2:stack-list-frames: use file= if fullname= is missing

When Python Frame Filters are used, GDB doesn't include a fullname=
field in its -stack-list-frames response record. Fall back to the value
of file= instead of reporting Unknown Source.

Fixes #1155

Author: malsyned

src/backend/mi2/mi2.ts

@@ -833,7 +833,7 @@ export class MI2 extends EventEmitter implements IBackend {
                     ret.push({
                         address: addr,
                         fileName: filename,
-                        file: file,
+                        file: file || filename,
                         function: func || from,
                         level: level,
                         line: line