Add a testcase for invalid OFF file

Testcase generated by AFL which used to crash the OFF loader

test/models/OFF/invalid.off

@@ -0,0 +1,8 @@
+OFF
+3 4
+0.1 0.2 0.3
+0.2 0.3 0.4
+0.3 0.4 0.5
+0
+2
20
+4