Declarator::clear(): Null out variable after it's been deleted.

This avoids a double free (which is good:-)

Bug submitted by Eli.

llvm-svn: 46105
diff --git a/clang/test/Sema/declspec.c b/clang/test/Sema/declspec.c
index d9e02ec..05f492d 100644
--- a/clang/test/Sema/declspec.c
+++ b/clang/test/Sema/declspec.c
@@ -3,3 +3,6 @@
 
 T foo(int n, int m) {  }  // expected-error {{cannot return array or function}}
 
+void foof(const char *, ...) __attribute__((__format__(__printf__, 1, 2))), barf (void);
+
+