Prevent invalid memory read in utils_scan_unicode_bom() when text
length is < 4.
Move utils_scan_unicode_bom(), utils_is_unicode_charset() to
encodings.c.
Read the BOM length in handle_bom().


git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@1167 ea778897-0a13-0410-b9d1-a72fbfd435f5