return empty completion lines on unsupported `COMP_POINT` (#76)

When `COMP_POINT` is invalid or out of range, return an empty list of
completions instead of raising `ValueError`. This matches behaviour for
`comp_point < 0 or comp_point > len(comp_line)` cases.

confutils/shell_completion.nim

@@ -120,7 +120,11 @@ proc getTok(l: var ShellLexer): Option[string] =
 
 proc splitCompletionLine*(): seq[string] =
   let comp_line = os.getEnv("COMP_LINE")
-  var comp_point = parseInt(os.getEnv("COMP_POINT", "0"))
+  var comp_point =
+    try:
+      parseInt(os.getEnv("COMP_POINT", "0"))
+    except ValueError:
+      return @[]
 
   if comp_point == len(comp_line):
     comp_point -= 1