release large (non ref) scalars using `undef $sv'

Using `undef EXPR' like a function call actually frees the heap
memory associated with the scalar, whereas `$sv = undef' or
`$sv = ""' will hold the buffer around until $sv goes out
of scope.

The `sv_set_undef' documentation in the perlapi(1) manpage
explicitly states this:

  The perl equivalent is "$sv = undef;". Note that it doesn't
  free any string buffer, unlike "undef $sv".

And I've confirmed by reading Dump() output from Devel::Peek.

We'll also inline the old index_body sub in SearchIdx.pm to make
the scope of the scalar more obvious.

This change saves several hundred kB RSS on both -index and
-httpd when hitting large emails with thousands of lines.

0 files changed, 0 insertions, 0 deletions