Update encodings.nim, fix open with bad arg raising no `EncodingErr…

…or` (#23481)

On POSIX, `std/encodings` uses iconv, and `iconv_open` returns
`(iconv_t) -1` on failure, not `NULL`

lib/pure/encodings.nim

@@ -342,7 +342,7 @@ proc open*(destEncoding = "UTF-8", srcEncoding = "CP1252"): EncodingConverter =
  ## Raises `EncodingError` if it cannot fulfill the request.
  when not defined(windows):
  result = iconvOpen(destEncoding, srcEncoding)
- if result == nil:
+ if result == cast[EncodingConverter](-1):
  raise newException(EncodingError,
  "cannot create encoding converter from " &
  srcEncoding & " to " & destEncoding)

tests/stdlib/tencodings.nim

@@ -101,3 +101,7 @@ block:
  doAssert orig == "\195\182\195\164\195\188\195\159"
  doAssert ibm850 == "\148\132\129\225"
  doAssert convert(ibm850, current, "ibm850") == orig
+
+block: # fixes about #23481
+ doAssertRaises EncodingError:
+ discard open(destEncoding="this is a invalid enc")