fix(postgres): fix buffer management in PgCopyIn.read_from (#2630)

sqlx-core/src/net/socket/buffered.rs

@@ -151,7 +151,7 @@ impl WriteBuffer {
             self.buf.truncate(self.bytes_written);
             self.buf.extend_from_slice(slice);
         }
-
+        self.advance(slice.len());
         self.sanity_check();
     }