SystemTools: Fix DetectFileType failure on missing file

Fix the following error reported by valgrind memcheck tool:

==31549== 256 bytes in 1 blocks are definitely lost in loss record 1 of 1
==31549==    at 0x4C2B800: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==31549==    by 0x44F3AA: kwsys::SystemTools::DetectFileType(char const*, unsigned long, double) (in .../kwsysTestsCxx)
==31549==    by 0x43D75E: CheckFileOperations() (in .../kwsysTestsCxx)
==31549==    by 0x43FAC5: testSystemTools(int, char**) (in .../kwsysTestsCxx)
==31549==    by 0x434A2B: main (in .../kwsysTestsCxx)

Change-Id: I42db9774bf30889b67921a538e00c1bf519a2719