[clang-tidy] fix a crash with -fdelayed-template-parsing in UnnecessaryValueParamCheck.

Summary:
This is the same kind of bug than [[ http://reviews.llvm.org/D18238 | D18238 ]].

Fix crashes caused by deferencing null pointer when declarations parsing may be delayed.
The body of the declarations may be null.

The crashes were observed with a Windows build of clang-tidy and the following command-line.
```
command-line switches: -fms-compatibility-version=19 -fms-compatibility
```

Reviewers: alexfh

Subscribers: kimgr, LegalizeAdulthood, cfe-commits

Differential Revision: http://reviews.llvm.org/D18852

llvm-svn: 265681