Ceph

when bufferlist len is 0, rebuild would new a prt with len 0 and insert into bufferlist.

The follow test case would assert.

bufferlist bl;
char t1[] = "X";
bufferlist a2;
a2.append(t1, 1);
bl.rebuild(); // this would insert new ptr with len 0
bl.append(a2);
bufferlist::iterator p = bl.begin();
char dst¹;
p.copy(1, dst); // assert