[MEMALLOC] Avoid gcc warning: variable 'oldstackp' set but not used

* src/memalloc.c (growstackblock): Remove declaration and set of
set-but-not-used variable.  Also remove a stray space-before-TAB.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>