Fix memory leak in portabtest Check_strtok_r()

Fix the following Clang "LeakSanitizer" error (which isn't quite
relevant in this test program, but anyway):

  ERROR: LeakSanitizer: detected memory leaks

  Direct leak of 7 byte(s) in 1 object(s) allocated from:
      #0 0x7f8c4d022810 in strdup (/lib/x86_64-linux-gnu/libasan.so.5+0x3a810)
      #1 0x5601a801491a in Check_strtok_r (/net/arthur/home/alex/Develop/ngIRCd/ngIRCd.git/src/portab/portabtest+0x291a)
      #2 0x5601a8014d77 in main (/net/arthur/home/alex/Develop/ngIRCd/ngIRCd.git/src/portab/portabtest+0x2d77)
      #3 0x7f8c4c69009a in __libc_start_main ../csu/libc-start.c:308

  SUMMARY: AddressSanitizer: 7 byte(s) leaked in 1 allocation(s).
  FAIL: portabtest