MBUG#688404 : Fix pbxt crashes on Windows 64 in debug build

The reason for the crash is misalignment on SSE instruciton
in setjmp(). The root cause is PBXT debug malloc(),  which
unlike  OS malloc  does not guarantee 16 bytes alignment.

So the fix for now is disable PBXT debug malloc on Windows.
It was obsolete anyway, as  it does not provide additional
benefits to C runtime debug routines (always used in debug 
compilation) or to pageheap, available at runtime.

storage/pbxt/src/memory_xt.h

@@ -29,8 +29,21 @@
 
 struct XTThread;
 
-#ifdef DEBUG
-#define DEBUG_MEMORY
+#if (defined DEBUG)
+/*
+	Disable PBXT debug malloc on Windows, as it is not properly aligned.
+	malloc() alignment requiremebt on x64 is documented as 16 bytes. PBXT debug
+	malloc is only 8 bytes aligned. Improper alignment will lead to a crash if
+	e.g SSE instructions access heap memory.
+
+	This might be general problem , however crashes were seen so far only
+	on Windows (crash during setjmp() on memory allocated with pbxt debug malloc).
+	
+	Besides, on Windows there is already a debug malloc by C runtime.
+*/
+#ifndef _WIN32
+	#define DEBUG_MEMORY
+#endif
 #endif
 
 #ifdef DEBUG_MEMORY