suppress a few compile warnings

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/lineedit.c

@@ -99,7 +99,7 @@ static bool BB_ispunct(CHAR_T c)
 # if ENABLE_FEATURE_EDITING_VI
 static bool BB_isalnum_or_underscore(CHAR_T c)
 {
-	return ((unsigned)c < 256 && isalnum(c)) || c == '_';
+	return isalnum(c) || c == '_';
 }
 # endif
 # define BB_ispunct(c) ispunct(c)