fix bug in cython code

Cython can not handle negative indices ( because of a derictive at the begining of the function that says not to do boundschecks). This is used to make the code faster, so this workaround is better then removing the directive.

fix bug in cython code
77a05341 · Razvan Pascanu · 1b4988c6 · 77a05341
--- a/theano/scan_module/scan_perform.pyx
+++ b/theano/scan_module/scan_perform.pyx
@@ -455,7 +455,8 @@ def perform(
            # if optimization gets applied compared to when optimization
            # do not get applied
            if i < n_steps:
-                outs[idx][0] = outs[idx][0][:-(n_steps - i)]
+                sh0 = outs[idx][0].shape[0]
+                outs[idx][0] = outs[idx][0][:sh0-(n_steps - i)]
    t_call = time.time() - t0_call