FIXED: Avoid corruption in setjmp()/longjmp()

Clang demands using a volatile variable for protecting the throw
environment's parent.  This is probably correct, although it seems
weird to place this variable in a register.

src/pl-incl.h

@@ -1800,8 +1800,8 @@ stack overflow.
 #define THROW_MAGIC 42424242
 
 typedef struct exception_frame		/* PL_throw exception environments */
-{ struct exception_frame *parent;	/* parent frame */
-  int		magic;			/* THROW_MAGIC */
+{ struct exception_frame * volatile parent;	/* parent frame */
+  volatile int	  magic;			/* THROW_MAGIC */
   jmp_buf	exception_jmp_env;	/* longjmp environment */
 } exception_frame;