Always call GC_on_mark_stack_empty when mark stack top reached

(fix of commit 0e63b39)

The previous check for GC_mark_stack_too_small did not indicated there
was not any room, rather that it was too small at some point during
marking.  If the stack had any content, it would set GC_mark_state to
MS_NONE and call alloc_mark_stack(), rather than call on_ms_empty()
first.

Without this change, GC_on_mark_stack_empty procedure would not be
executed when GC_mark_stack_too_small was set at any point during
a mark phase, e.g. in the MS_PUSH_RESCUERS case above.

* mark.c (GC_mark_some_inner): If on_ms_empty is non-zero, then call
on_ms_empty() even if GC_mark_stack_too_small (i.e. check
GC_mark_stack_too_small and call alloc_mark_stack() after on_ms_empty()
call); update comment.

mark.c

@@ -450,24 +450,20 @@ GC_API GC_on_mark_stack_empty_proc GC_CALL GC_get_on_mark_stack_empty(void)
             if ((word)GC_mark_stack_top >= (word)GC_mark_stack) {
                 MARK_FROM_MARK_STACK();
             } else {
-                GC_on_mark_stack_empty_proc on_ms_empty;
+                GC_on_mark_stack_empty_proc on_ms_empty =
+                                                GC_on_mark_stack_empty;
 
-                if (GC_mark_stack_too_small) {
-                    GC_mark_state = MS_NONE;
-                    alloc_mark_stack(2*GC_mark_stack_size);
-                    return TRUE;
-                }
-                on_ms_empty = GC_on_mark_stack_empty;
                 if (on_ms_empty != 0) {
                     GC_mark_stack_top = on_ms_empty(GC_mark_stack_top,
                                                     GC_mark_stack_limit);
-                    /* If we pushed new items or overflowed the stack,  */
-                    /* we need to continue processing.                  */
-                    if ((word)GC_mark_stack_top >= (word)GC_mark_stack
-                            || GC_mark_stack_too_small)
+                    /* If we pushed new items, we need to continue  */
+                    /* processing.                                  */
+                    if ((word)GC_mark_stack_top >= (word)GC_mark_stack)
                         break;
                 }
-
+                if (GC_mark_stack_too_small) {
+                    alloc_mark_stack(2*GC_mark_stack_size);
+                }
                 GC_mark_state = MS_NONE;
                 return TRUE;
             }