[BSDArchive] NULL check the child object file ptr before accessing its member

Recently we've observed lldb crashes caused by missing object file linked to a thin archive (.a) files. The crash is due to a missing NULL check in the code when looking for child object file referred by the thin archive. Malformed archive file should not crash LLDB. Instead, it should report the error and continue.

New error message will look like the following

```
error: libfoo.a(__objects__/foo/barAppDelegate.mm.o) failed to load objfile for path/to/libfoo.a.
Debugging will be degraded for this module.
```

Test Plan:

llvm-lit test
```
./bin/llvm-lit -sv ../llvm-project/lldb/test/API/functionalities/archives/TestBSDArchives.py
```

Test without code change will error out with LLDB crash
```
--
Command Output (stderr):
--
PASS: LLDB (~/llvm-upstream/Debug/bin/clang-arm64) :: test (TestBSDArchives.BSDArchivesTestCase)
PASS: LLDB (~/llvm-upstream/Debug/bin/clang-arm64) :: test_frame_var_errors_when_archive_missing (TestBSDArchives.BSDArchivesTestCase)
FAIL: LLDB (~/llvm-upstream/Debug/bin/clang-arm64) :: test_frame_var_errors_when_mtime_mistmatch_for_object_in_archive (TestBSDArchives.BSDArchivesTestCase)
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.      HandleCommand(command = "b a")
1.      HandleCommand(command = "breakpoint set --name 'a'")
Fatal Python error: Segmentation fault

Current thread 0x00000001f7b99e00 (most recent call first):
  File "~/llvm-upstream/Debug/bin/LLDB.framework/Resources/Python/lldb/__init__.py", line 3270 in HandleCommand
  File "~/llvm-upstream/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 2070 in runCmd
  File "~/llvm-upstream/llvm-project/lldb/packages/Python/lldbsuite/test/lldbtest.py", line 2421 in expect
  File "~/llvm-upstream/llvm-project/lldb/test/API/functionalities/archives/TestBSDArchives.py", line 156 in test_frame_var_errors_when_thin_archive_malformed
...
```

Differential Revision: https://reviews.llvm.org/D156367