Fix use-of-uninitialized value when RWX memory can't be allocated (PR6701).

Fix use-of-uninitialized value when RWX memory can't be allocated (PR6701).

SELinux doesn't allow 'execmem', returning MAP_FAILED and 'Permission denied'
for mmap or RWX memory. In this case AllocateRWX was returning a MemoryBlock
with uninitialized fields, which sometimes caused crashes.

This patch initializes MemoryBlock fields to 0, so that the RWX-failure check
works.
It doesn't fix the SELinux 'execmem' issues though (the JIT will not work when
SELinux is in enforcing mode).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99762 91177308-0d34-0410-b5e6-96231b3b80d8