Common: Fix a potential infinite loop in ReplaceAll

Prior to this change, it was possible to cause an infinite loop by making the string to be replaced and the replacing string the same thing.

e.g.

std::string some_str = "test";
ReplaceAll(some_str, "test", "test");

This also changes the replacing in a way that doesn't require starting from the beginning of the string on each replacement iteration.
Merge pull request #983 from lioncash/lol-str

Common: Fix a potential infinite loop in ReplaceAll