Combine two Adler-32 checksums into one.
adler1
Adler-32 checksum for sequence A
adler2
Adler-32 checksum for sequence B
len2
Length of sequence B
Given the Adler-32 checksum 'adler1' of a sequence A and the
Adler-32 checksum 'adler2' of a sequence B of length 'len2',
return the Adler-32 checksum of A and B concatenated.