Merge bitcoin/bitcoin#24297: Fix unintended unsigned integer overflow in strencodings

fac9fe5 Fix unintended unsigned integer overflow in strencodings (MarcoFalke)

Pull request description:

  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.

ACKs for top commit:
  laanwj:
    Code review ACK fac9fe5
  shaavan:
    Code Review ACK fac9fe5

Tree-SHA512: e71c21a0b617abf241e561ce6b90b963e2d5e2f77bd9547ce47209a1a94b454384391f86ef5d35fedd4f4df19add3896bb3d61fed396ebba8e864e3eeb75ed59

Author: fanquake

src/util/strencodings.cpp

@@ -113,7 +113,7 @@ void SplitHostPort(std::string in, uint16_t& portOut, std::string& hostOut)
     // if a : is found, and it either follows a [...], or no other : is in the string, treat it as port separator
     bool fHaveColon = colon != in.npos;
     bool fBracketed = fHaveColon && (in[0] == '[' && in[colon - 1] == ']'); // if there is a colon, and in[0]=='[', colon is not 0, so in[colon-1] is safe
-    bool fMultiColon = fHaveColon && (in.find_last_of(':', colon - 1) != in.npos);
+    bool fMultiColon{fHaveColon && colon != 0 && (in.find_last_of(':', colon - 1) != in.npos)};
     if (fHaveColon && (colon == 0 || fBracketed || !fMultiColon)) {
         uint16_t n;
         if (ParseUInt16(in.substr(colon + 1), &n)) {