mem: fix build with -O1

mem: fix build with -O1

When compiled with EXTRA_CFLAGS="-O1", the compiler is not
able to detect that size is always initialized when used, and
issues a wrong warning:

  eal_memory.c: In function ‘rte_eal_hugepage_attach’:
  eal_memory.c:1684:3: error: ‘size’ may be used uninitialized in this
                       function [-Werror=maybe-uninitialized]
     munmap(hp, size);
     ^

Workaround this issue by initializing size to 0.
Seen on gcc (Debian 5.4.1-1) 5.4.1 20160803.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>