Fix GC_is_visible for case of arg pointing exactly to object upper bound

(a cherry-pick of commit e8fa865 from 'master')

In the single-threaded configuration of the collector, if p points
exactly to base of the object plus its length then treat is a pointer
outside the object (this behavior is similar to that of a bitmap-based
descriptor).

* ptr_chck.c [!THREADS] (GC_is_visible): If descr is length-based and
p-base==descr then fail.

ptr_chck.c

@@ -231,7 +231,7 @@ GC_API void * GC_CALL GC_is_visible(void *p)
     retry:
             switch(descr & GC_DS_TAGS) {
                 case GC_DS_LENGTH:
-                    if ((word)p - (word)base > descr) goto fail;
+                    if ((word)p - (word)base >= descr) goto fail;
                     break;
                 case GC_DS_BITMAP:
                     if ((word)p - (word)base >= WORDS_TO_BYTES(BITMAP_BITS)