Bug #1107: Removed unnecessary null pointer checks

Explicit null pointer checks were performed at a few source code places
before pointers were passed to the C++ delete operator. This was unnecessary
because the delete operator will care for such parameter validation already.
http://www.parashift.com/c++-faq/delete-handles-null.html

Redundant checks were removed.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Merge pull request #1110 from elfring/Remove_unnecessary_null_pointer_checks

Remove unnecessary null pointer checks.