fix fallback code path in take!(::IOBuffer) method (JuliaLang#58762)

JET told me that the `data` local variable was inparticular is undefined
at this point.
After reviewing this code, I think this code path is unreachable
actually since `bytesavailable(io::IOBuffer)` returns `0` when `io` has
been closed. So it's probably better to make it clear.

Author: aviatesk

base/iobuffer.jl

@@ -771,7 +771,7 @@ function take!(io::IOBuffer)
         elseif io.writable
             data = wrap(Array, memoryref(io.data, io.ptr), nbytes)
         else
-            data = read!(io, data)
+            error("Unreachable IOBuffer state")
         end
     end
     if io.writable