Fix GH-15824 mb_detect_encoding() invalid "UTF8" (#15829)

I fixed from strcasecmp to strncasecmp.
However, strncasecmp is specify size to #3 parameter.
Hence, Add check length to mim

Fix GH-15824 mb_detect_encoding() invalid "UTF8" (#15829)

I fixed from strcasecmp to strncasecmp.
However, strncasecmp is specify size to #3 parameter.
Hence, Add check length to mime and aliases.

Co-authored-by: Niels Dossche <7771979+nielsdos@users.noreply.github.com>

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