sys/tree.h: Simplify loop condition

We have

  #define RB_ISRED(elm, field) \
    ((elm) != NULL && RB_COLOR(elm, field) == RB_RED)

So, the RB_ISRED() contains an implicit check for NULL.  In
RB_GENERATE_REMOVE_COLOR() the "elm" pointer cannot be NULL in the while
condition.  Use RB_COLOR(elm) == RB_BLACK instead.

newlib/libc/include/sys/tree.h

@@ -540,7 +540,7 @@ name##_RB_REMOVE_COLOR(struct name *head, struct type *parent)		\
 			elm = RB_ROOT(head);				\
 			break;						\
 		}							\
-	} while (!RB_ISRED(elm, field) && parent != NULL);		\
+	} while (RB_COLOR(elm, field) == RB_BLACK && parent != NULL);	\
 	RB_COLOR(elm, field) = RB_BLACK;				\
 }