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).

llvm-svn: 99762

llvm/include/llvm/System/Memory.h

@@ -27,7 +27,7 @@ namespace sys {
   /// @brief Memory block abstraction.
   class MemoryBlock {
   public:
-    MemoryBlock() { }
+    MemoryBlock() : Address(0), Size(0) { }
     MemoryBlock(void *addr, size_t size) : Address(addr), Size(size) { }
     void *base() const { return Address; }
     size_t size() const { return Size; }