[boost] [mp_int] Patch for deallocate(NULL)

In the version of mp_math posted to the vault in Oct 2008 there are several calls to std::allocator::deallocate where the argument pointer can be NULL. This is not compliant with the C++ standard, which states "p must not be null" (Table 6 in 20.1.5 in the Draft Standard that I have access to). Sure enough, when p is null, I get a segmentation fault (using GCC 3.2.3). Attached is a patch with a fix. -- Nathan