(cantwrite): do not allocate memory for a NULL string

PR:		misc/26044

MFC after:	1 week

Submitted by:	bde

lib/libc/stdio/local.h

@@ -81,7 +81,8 @@ struct __sFILEX {
  * Return true iff the given FILE cannot be written now.
  */
 #define	cantwrite(fp) \
-	((((fp)->_flags & __SWR) == 0 || (fp)->_bf._base == NULL) && \
+ 	((((fp)->_flags & __SWR) == 0 || \
+ 	    ((fp)->_bf._base == NULL && ((fp)->_flags & __SSTR) == 0)) && \
 	 __swsetup(fp))
 
 /*