Fix a memory leak in ContextImpl

A typo in the Teardown code caused it to not
destroy the cusolver handles, and thus leak memory.

Change-Id: Id52661dd58dcd0356027e350ba78d381484dc2b6

internal/ceres/context_impl.cc

diff --git a/internal/ceres/context_impl.cc b/internal/ceres/context_impl.cc
index 0ad2abe..56c9785 100644
--- a/internal/ceres/context_impl.cc
+++ b/internal/ceres/context_impl.cc
@@ -56,7 +56,7 @@
     cublasDestroy(cublas_handle_);
     cublas_handle_ = nullptr;
   }
-  if (cusolver_handle_ != nullptr) {
+  if (cusparse_handle_ != nullptr) {
     cusparseDestroy(cusparse_handle_);
     cusparse_handle_ = nullptr;
   }