Strengthen a few assertions and add zapping of allocated...

src/spaces.h

Index: src/spaces.h
===================================================================
--- src/spaces.h	(revision 3725)
+++ src/spaces.h	(working copy)
@@ -438,13 +438,16 @@
   // and false otherwise.
   static bool CommitBlock(Address start, size_t size, Executability executable);
 
-
   // Uncommit a contiguous block of memory [start..(start+size)[.
   // start is not NULL, the size is greater than zero, and the
   // block is contained in the initial chunk.  Returns true if it succeeded
   // and false otherwise.
   static bool UncommitBlock(Address start, size_t size);
 
+  // Zaps a contiguous block of memory [start..(start+size)[ thus
+  // filling it up with a recognizable non-NULL bit pattern.
+  static void ZapBlock(Address start, size_t size);
+
   // Attempts to allocate the requested (non-zero) number of pages from the
   // OS.  Fewer pages might be allocated than requested. If it fails to
   // allocate memory for the OS or cannot allocate a single page, this