Fix end iterator dereference with specific util::strftime() input

A format string with a trailing '%' character at the end would cause an
implementation detail to dereference the end iterator of a string. Since
the underlying storage is a null-terminated string, this would be
harmless in practice.

Found by cppcheck.

src/strftime.cpp

@@ -48,6 +48,11 @@ static std::string reformat(const std::string& format, const std::tm* time,
 		if (*it == '%') {
 			++it;
 
+			if(it == format.end()) {
+				new_format += "%";
+				break;
+			}
+
 			bool unrecognized = false;
 
 			if (!locale_correct) {