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.