Assume p is not nullptr in storeUnaligned

Assume p is not nullptr in storeUnaligned

Summary:
Because we're constructing the value with a placement new, which has some of C++'s most unhelpful behavior ever put into the spec: If `p` is `nullptr` and we are not compiling in C++14 mode, where the restriction was changed, then the placement new will do absolutely nothing at all. By adding the assumption that `p` is not `nullptr`, we'll trip a segfault in release mode rather than failing silently.
Note that MSVC would generate the nullptr check regardless of which mode it's in, so this assume forces the removal of the check.

Reviewed By: yfeldblum

Differential Revision: D3651116

fbshipit-source-id: ee15a38f85ce4e3cb3186fda0b7bcca39acda27a