Add guard pages in front of platform allocations

src/platform-macos.cc

Index: src/platform-macos.cc
===================================================================
--- src/platform-macos.cc	(revision 8619)
+++ src/platform-macos.cc	(working copy)
@@ -148,7 +148,7 @@
                    bool is_executable) {
   const size_t msize = RoundUp(requested, getpagesize());
   int prot = PROT_READ | PROT_WRITE | (is_executable ? PROT_EXEC : 0);
-  void* mbase = mmap(NULL, msize, prot,
+  void* mbase = mmap(NULL, msize + Page::kPageSize, prot,
                      MAP_PRIVATE | MAP_ANON,
                      kMmapFd, kMmapFdOffset);
   if (mbase == MAP_FAILED) {
@@ -156,6 +156,8 @@
     return NULL;
   }
   *allocated = msize;
+  mprotect(mbase, Page::kPageSize, PROT_NONE);
+  mbase += Page::kPageSize;
   UpdateAllocatedSpaceLimits(mbase, msize);
   return mbase;
 }
@@ -163,7 +165,7 @@
 
 void OS::Free(void* address, const size_t size) {
   // TODO(1240712): munmap has a return value which is ignored here.
-  int result = munmap(address, size);
+  int result = munmap(address - Page::kPageSize, size + Page::kPageSize);
   USE(result);
   ASSERT(result == 0);
 }