MINOR: debug: make the ABORT_NOW macro use a volatile int

Similar to previous commit, let's make the macro use a volatile when
dereferencing NULL so that clang doesn't optimize it away.

include/common/debug.h

@@ -40,7 +40,7 @@
 /* This abort is more efficient than abort() because it does not mangle the
  * stack and stops at the exact location we need.
  */
-#define ABORT_NOW() (*(int*)0=0)
+#define ABORT_NOW() (*(volatile int*)0=0)
 
 /* this one is provided for easy code tracing.
  * Usage: TRACE(strm||0, fmt, args...);