Fix integer overflow in GenerateInt32SeedFromString.
Fix integer overflow in GenerateInt32SeedFromString.

This is reported as a multiplication overflow with Clang and
-fsanitize=undefined.

google/or-tools,legrosbuffle/or-tools,or-tools/or-tools,or-tools/or-tools,legrosbuffle/or-tools,google/or-tools,ThatRfernand/or-tools,ThatRfernand/or-tools,ThatRfernand/or-tools,google/or-tools,or-tools/or-tools,legrosbuffle/or-tools,legrosbuffle/or-tools,google/or-tools,google/or-tools,google/or-tools,or-tools/or-tools,or-tools/or-tools,ThatRfernand/or-tools,or-tools/or-tools,ThatRfernand/or-tools,legrosbuffle/or-tools