BUG/MINOR: cache: properly ignore unparsable max-age in quotes

BUG/MINOR: cache: properly ignore unparsable max-age in quotes

When "max-age" or "s-maxage" receive their values in quotes, the pointer
to the integer to be parsed is advanced by one, but the error pointer
check doesn't consider this advanced offset, so it will not match a
parse error such as max-age="a" and will take the value zero instead.

This probably needs to be backported, though it's unsure it has any
effect in the real world.