[clang-tidy] clang-apply-replacements: Don't insert null entry

[clang-tidy] clang-apply-replacements: Don't insert null entry

Summary:
[clang-tidy] clang-apply-replacements: Don't insert null entry

Fix crash when running clang-apply-replacements on YML files which
contain an invalid file path. Make sure we never add a nullptr into the
map. The previous code started adding nullptr to the map after the first
warnings via errs() has been emitted.

Backtrace:
```
Starting program:
/home/kfunk/devel/build/llvm/bin/clang-apply-replacements /tmp/tmpIqtp7m
[Thread debugging using libthread_db enabled]
Using host libthread_db library
"/lib/x86_64-linux-gnu/libthread_db.so.1".
Described file '.moc/../../../../../../src/qt5.8/qtremoteobjects/src/remoteobjects/qremoteobjectregistrysource_p.h' doesn't exist. Ignoring...
...

Program received signal SIGSEGV, Segmentation fault.
main (argc=<optimized out>, argv=<optimized out>) at /home/kfunk/devel/src/llvm/tools/clang/tools/extra/clang-apply-replacements/tool/ClangApplyReplacementsMain.cpp:262
(gdb) p FileAndReplacements.first
$1 = (const clang::FileEntry *) 0x0
(gdb)
```

Added tests.

Before patch:

```
******************** TEST 'Clang Tools :: clang-apply-replacements/invalid-files.cpp' FAILED ********************
Script:
--
mkdir -p /home/kfunk/devel/build/llvm/tools/clang/tools/extra/test/clang-apply-replacements/Output/Inputs/invalid-files
clang-apply-replacements /home/kfunk/devel/build/llvm/tools/clang/tools/extra/test/clang-apply-replacements/Output/Inputs/invalid-files
ls -1 /home/kfunk/devel/build/llvm/tools/clang/tools/extra/test/clang-apply-replacements/Output/Inputs/invalid-files | FileCheck /home/kfunk/devel/src/llvm/tools/clang/tools/extra/test/clang-apply-replacements/invalid-files.cpp --check-prefix=YAML
--
Exit Code: 139

Command Output (stderr):
--
Described file 'idonotexist.h' doesn't exist. Ignoring...
/home/kfunk/devel/build/llvm/tools/clang/tools/extra/test/clang-apply-replacements/Output/invalid-files.cpp.script: line 4:  9919 Segmentation fault      clang-apply-replacements /home/kfunk/devel/build/llvm/tools/clang/tools/extra/test/clang-apply-   replacements/Output/Inputs/invalid-files

--
```

After Patch:

```
PASS: Clang Tools :: clang-apply-replacements/invalid-files.cpp (5 of 6)
```

Reviewers: alexfh, yawanng

Reviewed By: alexfh

Subscribers: cfe-commits, klimek, JDevlieghere, xazax.hun

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D35194

llvm-svn: 308974