do not segfault if error is not set by libgit2

src/pygit2/error.c

@@ -33,6 +33,7 @@ PyObject * Error_type(int type)
 
     // Critical
     const git_error* error = giterr_last();
+    if(error != NULL) {
         switch (error->klass) {
             case GITERR_NOMEMORY:
                 return PyExc_MemoryError;
@@ -40,9 +41,9 @@ PyObject * Error_type(int type)
                 return PyExc_OSError;
             case GITERR_INVALID:
                 return PyExc_ValueError;
-        default:
-            return GitError;
         }
+    }
+    return GitError;
 }
 
 
@@ -54,7 +55,10 @@ PyObject* Error_set(int err)
         PyErr_SetNone(Error_type(err));
     } else { //critical failure
         const git_error* error = giterr_last();
-      PyErr_SetString(Error_type(err), error->message);
+        char* message = (error == NULL) ? 
+                "(No error information given)" : error->message;
+        
+        PyErr_SetString(Error_type(err), message);
     }
 
     return NULL;