Do not do iterator underflow in urlDecode

`value.end() - 3` may be "before start of string" if string length is < 3.
On Windows debug, is throw an exception.
On other platform it is probably an undefined behavior.

Rewrite the test to avoid such invalid substraction.

src/tools/stringTools.cpp

@@ -257,7 +257,7 @@ std::string kiwix::urlDecode(const std::string& value, bool component)
 
     // If there aren't enough characters left for this to be a
     // valid escape code, just use the character and move on
-    if (it > value.end() - 3) {
+    if (value.end() - it < 3) {
       os << *it;
       continue;
     }