Fix overflow checking for repeat operation

[marco6]

The current overflow check for repeat operation fails to check when a multiplication wraps to a positive number.

On 64 bits build it is hard to get one (as you would need a tuple/string with more than 2^32 element - but it's still possible).

On a 32 bits build however it is quite trivial:

# tuples and lists return the wrong count
len((1,) * 641 * 6700417) # returns 1
len([1] * 641 * 6700417) # returns 1

# strings/bytes cause panic: strings: Repeat count causes overflow
"a" * 641 * 6700417

This PR uses bits.Mul to make sure that this does not happen.

[adonovan]

Well spotted! Many thanks for contributing this fix.