[MLlib] Squash bug in IndexedRowMatrix

Kill this bug fast before it does damage.

Author: Reza Zadeh <rizlar@gmail.com>

Closes #2224 from rezazadeh/indexrmbug and squashes the following commits:

53386d6 [Reza Zadeh] Squash bug in IndexedRowMatrix

(cherry picked from commit 0f16b23cd17002fac05f3ecc58899be1b1121b82)
Signed-off-by: Xiangrui Meng <meng@databricks.com>