Add _REENT_IS_NULL()

In a follow up patch, struct _reent is optionally replaced by dedicated
thread-local objects.  In this case,_REENT is optionally defined to NULL.  Add
the _REENT_IS_NULL() macro to disable this check on demand.

newlib/libc/include/sys/reent.h

@@ -763,6 +763,8 @@ extern struct _reent _impure_data __ATTRIBUTE_IMPURE_DATA__;
 # define _REENT _impure_ptr
 #endif /* __SINGLE_THREAD__ || !__DYNAMIC_REENT__ */
 
+#define _REENT_IS_NULL(_ptr) ((_ptr) == NULL)
+
 #define _GLOBAL_REENT (&_impure_data)
 
 /* This value is used in stdlib/misc.c.  reent/reent.c has to know it

newlib/libc/machine/spu/c99ppe.h

@@ -104,6 +104,12 @@ FILE  *__sfp (struct _reent *);
 #define __sfp_free(fp) ( (fp)->_fp = 0 )
 
 #define CHECK_INIT(ptr) \
-  do { if ((ptr) && !_REENT_CLEANUP(ptr)) __sinit (ptr); } while (0)
+  do						\
+    {						\
+      if (!_REENT_IS_NULL(ptr) &&		\
+	  !_REENT_CLEANUP(ptr))			\
+	__sinit (ptr);				\
+    }						\
+  while (0)
 #define CHECK_STR_INIT(ptr) /* currently, do nothing */
 #endif /* __ASSEMBLER__ */

newlib/libc/stdio/local.h

@@ -197,7 +197,8 @@ extern _READ_WRITE_RETURN_TYPE __swrite64 (struct _reent *, void *,
   do								\
     {								\
       struct _reent *_check_init_ptr = (ptr);			\
-      if ((_check_init_ptr) && !_REENT_CLEANUP(_check_init_ptr))\
+      if (!_REENT_IS_NULL(_check_init_ptr) &&			\
+	  !_REENT_CLEANUP(_check_init_ptr))			\
 	__sinit (_check_init_ptr);				\
     }								\
   while (0)

newlib/libc/sys/arm/syscalls.c

@@ -60,7 +60,8 @@ extern void   __sinit (struct _reent *);
 #define CHECK_INIT(ptr) \
   do						\
     {						\
-      if ((ptr) && !_REENT_CLEANUP(ptr))	\
+      if (!_REENT_IS_NULL(ptr) &&		\
+	  !_REENT_CLEANUP(ptr))			\
 	__sinit (ptr);				\
     }						\
   while (0)