[libFuzzer] Update Load test to work on 32 bits.

We should ensure the size of the variable `a` is 8 bytes. Otherwise, this
generates a stack buffer overflow inside the memcpy call in 32 bits machines.
(We write more bytes than the size of a, when it is 4 bytes)

Differential Revision: https://reviews.llvm.org/D29602

llvm-svn: 294378