Memory leak in locally evalutated expressions during SP execution fixed by
  reusing allocated item slots when possible.
  Note: No test case added, since the test is a stress test that tries to make
  the machine to run out of memory.
  Second attempt, now tested with debug build, valgrind build, max (optimized)
  build, with and without --debug, --vagrind and --ps-protocol.
  Errors in trigger and view test with --debug in debug build where present
  before this patch, and likewise for valgrind warnings for view test in
  valgrind build with --ps-protocol.