Avoid doing arithmetic with NULL

Spotted by Florian Krohm/BEAM



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14515 a5019735-40e9-0310-863c-91ae7b9d1cf9
Rename VG_(malloc_usable_size) to VG_(cli_malloc_usable_size)
because it operates on the CLIENT arena. Given that VG_(malloc)
operates on the CORE arena, it was unexpected for VG_(mallos_usable_size)
to use a different arena. 
Move function definition to the proper place (next to VG_(cli_malloc))
and fix call sites.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14516 a5019735-40e9-0310-863c-91ae7b9d1cf9