Return position of first un-encodable character in a region.
START and END specify the region and CODING-SYSTEM specifies the
encoding to check. Return nil if CODING-SYSTEM does encode the region.
If optional 4th argument COUNT is non-nil, it specifies at most how
many un-encodable characters to search. In this case, the value is a
list of positions.
If optional 5th argument STRING is non-nil, it is a string to search
for un-encodable characters. In that case, START and END are indexes
to the string.
(fn START END CODING-SYSTEM &optional COUNT STRING)