Fix typo of SparseMat_<_Tp>::SparseMat_(const SparseMat& m)

Fix compilation erros when compiling this constructor.
First argument type of "convertTo" should be instance, not a pointer of instance.

First pull request was created for master branch.
But it should be marged for 2.4.
Old pull request -> https://github.com/Itseez/opencv/pull/2113