Instead calculate a hash based on the input and use that as a seed
into random data generation for the target env.
Also fixes issue where input data was not actually being fed into
one fuzzer.
Fixes#4450
Pending a more general solution for constructing a target environment
based on the bytes of a test input, this change avoids a UBSan error
caused by the existing approach.
Fixes https://crbug.com/38087
