[issue29838] Check that sq_length and mq_length return non-negative result

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

assert() is no-op in release build and that are not critical parts.

Actually the patch decreases the number of checks in release build. Runtime check PyErr_Occurred() is moved into assert() in builtin_len().

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<http://bugs.python.org/issue29838>
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