Fix unintended unsigned integer overflow in strencodings #24297

This fixes two issues for strings that start with a colon and only have one colon:

• fMultiColon is incorrectly set to true
• There is an unsigned integer overflow colon - 1 (0 - 1)

Neither issue matters, as the result is discarded. Though, it makes sense to still fix the issue for clarity and to avoid sanitizer issues in the function.

Code review ACK fac9fe5d051264fcd16e8e36d30f28c05c999837

Code Review ACK fac9fe5d051264fcd16e8e36d30f28c05c999837

Though the result of this section is discarded, I think it makes to keep the code correct as it keeps the code logically sound, while also avoid risk of potential future bugs, due to this piece of code.