Fix libntlmauth string parsing on big-endian machines (#2242)

Fix libntlmauth string parsing on big-endian machines (#2242)

Prevent off-by-one reads in ntlm_fetch_string(); clamp copies too.
Convert flags with le32toh; validate lengths; ensure NUL-terminate.
cast ntlm string characters to unsigned char explicitly.
Return BlobError on oversized fields to avoid UB.

Combined these changes affect Big-Endian CPU architectures
which will fail to detect ASCII vs UTF encoding properly and
produce invalid strings (including user/password to compare).