`IOBuffer` `skip` regression

[nhz2]

#57570 broke skip for IOBuffer. This is causing TranscodingStreams.jl tests to fail on nightly.

MWE:

iob = IOBuffer(repeat("x", 400))
skip(iob, 10)
skip(iob, 400)
read(iob)

This throws:

ERROR: InexactError: convert(UInt64, -10)
Stacktrace:
  [1] throw_inexacterror(func::Symbol, to::Type, val::Int64)
    @ Core ./boot.jl:815
  [2] check_sign_bit
    @ ./boot.jl:821 [inlined]
  [3] toUInt64
    @ ./boot.jl:932 [inlined]
  [4] UInt64
    @ ./boot.jl:962 [inlined]
  [5] convert
    @ ./number.jl:7 [inlined]
  [6] _string_n
    @ ./strings/string.jl:109 [inlined]
  [7] StringMemory
    @ ./iobuffer.jl:167 [inlined]
  [8] StringVector
    @ ./iobuffer.jl:168 [inlined]
  [9] read(io::IOBuffer)
    @ Base ./iobuffer.jl:894
 [10] top-level scope
    @ REPL[22]:1