Add guard pages in front of platform allocations

src/platform-macos.cc

Index: src/platform-macos.cc
===================================================================
--- src/platform-macos.cc	(revision 8657)
+++ src/platform-macos.cc	(working copy)
@@ -169,6 +169,11 @@
 }
 
 
+void OS::Guard(void* address, const size_t size) {
+  mprotect(address, size, PROT_NONE);
+}
+
+
 void OS::Sleep(int milliseconds) {
   usleep(1000 * milliseconds);
 }