Strengthen assertions in fellow_busy_obj_trimstore()

assert that the FCS_BUSY segment to be trimmed is in fact contained in
the current body region.

src/fellow_cache.c

@@ -4271,6 +4271,21 @@ fellow_busy_obj_trim_seglists(struct fellow_busy *fbo)
 	fellow_cache_seglist_free(fbo->fc, fbo->body_seglist);
 }
 
+static inline int
+fdr_contains(
+    const struct buddy_off_extent *outer,
+    const struct buddy_off_extent *inner)
+{
+
+	assert(outer->off > 0);
+	assert(inner->off > 0);
+
+	return (
+	    inner->off >= outer->off &&
+	    (typeof(inner->size))inner->off + inner->size <=
+	    (typeof(outer->size))outer->off + outer->size);
+}
+
 /*
  * we need to write full disk blocks and if the membuddy minsize is less than
  * the dskbuddy minsize, we could trim to under the blocksize
@@ -4310,6 +4325,15 @@ fellow_busy_obj_trimstore(struct fellow_busy *fbo)
 	if (fcs->state == FCS_BUSY) {
 		assert(fcs->len <= fcs->alloc.size);
 		AN(fdr);
+
+		/*
+		 * fcs must be contained in the fdr, there can
+		 * be a free region behind it
+		 */
+
+		assert(fcs->disk_seg->seg.off > 0);
+		assert(fcs->disk_seg->seg.size > 0);
+		assert(fdr_contains(fdr, &fcs->disk_seg->seg));
 	} else {
 		assert(fcs->state == FCS_USABLE);
 		AZ(fcs->alloc.size);