libxml2: xmlcatalog safer memcpy (Patch Rewards)

[destro4evr-rgb]

Safer bounds-checked memcpy in xmlcatalog.c:108 prevents OOB.

Before: memcpy (ret, buf, len + 1); (overrun risk)
After: memcpy(ret, buf, len); ret[len] = 0;

Rel: Google Patch Rewards Tier 2 OSS-Fuzz https://bughunters.google.com/open-source-security/patch-rewards

CC @xml @Oss-fuzz

[destro4evr-rgb]

Submitted for Google Patch Rewards Program (Tier 2 OSS-Fuzz libxml2).
Prevents memcpy OOB in xmlcatalog parsing.

Thanks for review!