branches/5.1: Use btr_free_root() instead of fseg_free() for

the fix of bug #48469, because fseg_free() is not defined
in the zip branch. And we could save one mini-trasaction started
by fseg_free().

Approved by Marko.

btr/btr0btr.c

@@ -713,8 +713,7 @@ btr_create(
 				 PAGE_HEADER + PAGE_BTR_SEG_LEAF, mtr)) {
 			/* Not enough space for new segment, free root
 			segment before return. */
-			fseg_free(space, page_no,
-				  PAGE_HEADER + PAGE_BTR_SEG_TOP);
+			btr_free_root(space, page_no, mtr);
 
 			return(FIL_NULL);
 		}