fix internalError in checkFormatString on invalid width or position - #8982

CheckIOImpl::checkFormatString converts the digits collected for a field width or a POSIX parameter position with strToInt, which throws when the digits are empty (%$d), start with a zero (%01$d, which glibc accepts) or do not fit in an int (%99999999999$d, scanf("%99999999999s", buf) with a known array), so one such format string makes CppCheck::checkFile bail out with an internalError and drop every other diagnostic for that file. Replaced the conversion with a small helper that parses the digits without throwing and clamps anything above INT_MAX, and a $ with no digits in front is no longer taken as a parameter position; widths and positions that fit in an int behave exactly as before, while an oversized one is now reported through the existing invalidScanfFormatWidth and wrongPrintfScanfParameterPositionError messages, with 2147483647 shown for the clamped value. Verified with an ASan/UBSan build: the new asserts in TestIO::testScanf4 and testPosixPrintfScanfParameterPosition throw on main and pass with the fix, the full testrunner is green, and the selfcheck options report nothing for lib/checkio.cpp. I do not have Trac access, so could you create a ticket for this? I will update the title to reference it once there is a number.