asn.c: guard empty otherName copy to avoid NULL memcpy (UBSan)

[night1rider]

When a subjectAltName otherName has an empty value, SetDNSEntry() called XMEMCPY(dst, NULL, 0). Passing NULL to memcpy is undefined behavior and aborts under UBSan. Skip the copy unless there is real data to copy.

[github-actions]

MemBrowse Memory Report

gcc-arm-cortex-m0plus

• FLASH: .text +12 B (+0.0%, 64,363 B / 262,144 B, total: 25% used)

gcc-arm-cortex-m3

• FLASH: .text +8 B (+0.0%, 123,091 B / 262,144 B, total: 47% used)

gcc-arm-cortex-m4-tls12

• FLASH: .text +64 B (+0.1%, 123,891 B / 262,144 B, total: 47% used)
  No memory changes detected for:
• gcc-arm-cortex-m4
• gcc-arm-cortex-m4-baremetal
• gcc-arm-cortex-m4-crypto-only
• gcc-arm-cortex-m4-dtls13
• gcc-arm-cortex-m4-min-ecc
• gcc-arm-cortex-m4-openssl-compat
• gcc-arm-cortex-m4-pkcs7
• gcc-arm-cortex-m4-pq
• gcc-arm-cortex-m4-rsa-only
• gcc-arm-cortex-m4-sp-math
• gcc-arm-cortex-m4-tls13
• gcc-arm-cortex-m7
• gcc-arm-cortex-m7-pq
• gcc-arm-cortex-m7-tls13
• linuxkm-pie
• linuxkm-standard
• stm32-sim-stm32h753

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #11117

Scan targets checked: wolfcrypt-bugs, wolfcrypt-src

No new issues found in the changed files. ✅