ASN.1 date parser overread

libcurl's ASN1 parser code has the `GTime2str()` function, used for parsing an ASN.1 Generalized Time field. If given an syntactically incorrect field, the parser might end up using -1 for the length of the *time fraction*, leading to a `strlen()` getting performed on a pointer to a heap buffer area that is not (purposely) null terminated. This flaw most likely leads to a crash, but can also lead to heap contents getting returned to the application when [CURLINFO_CERTINFO](https://curl.se/libcurl/c/CURLINFO_CERTINFO.html) is used.