Fix equalsIdentity() for rectangular matrices.

test/nullary.cpp

diff --git a/test/nullary.cpp b/test/nullary.cpp
index dd42cb8..3a46e10 100644
--- a/test/nullary.cpp
+++ b/test/nullary.cpp
@@ -37,7 +37,7 @@
     }
   }
   for (int i = 0; i < A.rows(); ++i) {
-    for (int j = 0; j < i; ++j) {
+    for (int j = 0; j < std::min(i, A.cols()); ++j) {
       offDiagOK = offDiagOK && (A(i,j) == zero);
     }
   }