More NULL dereferences in x509asn1.c

[z2-2z]

Hello,

in #13972 you fixed a possible NULL dereference in the ASN1tostr() function.
The root cause was that when converting a Curl_asn1Element with length = 0 to a string,
the dynbuf temp in ASN1tostr() does not get initialized through
one path of the ASN1tostr() function such that Curl_dyn_ptr(&temp) then returns NULL.

Unfortunately there are at least 4 more paths through ASN1tostr() and its helper functions that lead to the same
outcome, involving the functions:

• utf8asn1str()
• octet2str()
• OID2str()

This PR tries to fix the NULL derefs in one place instead of all the helper functions above.

For reference, I attached all 4 certificates that cause a NULL dereference:
crash-ecf257c87027696ed7f041bb764e0028adb95408
crash-26e2227eeeac47626b14bedfa6793b7c85223fca
crash-da35663a566f4c1ccd929bb658b64e2ab925cd07
crash-fddfc08fd1b21a188f99692c8aaf85fdc560ef5d

[bagder]

Thanks!