Revert "Revert "Revert "Add implicit null and stack checks for x86"""

Broke the build.

This reverts commit 7fb36ded9cd5b1d254b63b3091f35c1e6471b90e.

Change-Id: I9df0e7446ff0913a0e1276a558b2ccf6c8f4c949
diff --git a/runtime/thread_linux.cc b/runtime/thread_linux.cc
index 518211b..ee66ccc 100644
--- a/runtime/thread_linux.cc
+++ b/runtime/thread_linux.cc
@@ -35,8 +35,8 @@
 void Thread::SetUpAlternateSignalStack() {
   // Create and set an alternate signal stack.
   stack_t ss;
-  ss.ss_sp = new uint8_t[SIGSTKSZ * 2];   // NB. this is 16K.
-  ss.ss_size = SIGSTKSZ * 2;
+  ss.ss_sp = new uint8_t[SIGSTKSZ];
+  ss.ss_size = SIGSTKSZ;
   ss.ss_flags = 0;
   CHECK(ss.ss_sp != NULL);
   SigAltStack(&ss, NULL);
@@ -56,7 +56,7 @@
   // Tell the kernel to stop using it.
   ss.ss_sp = NULL;
   ss.ss_flags = SS_DISABLE;
-  ss.ss_size = SIGSTKSZ * 2;  // Avoid ENOMEM failure with Mac OS' buggy libc.
+  ss.ss_size = SIGSTKSZ;  // Avoid ENOMEM failure with Mac OS' buggy libc.
   SigAltStack(&ss, NULL);
 
   // Free it.