suppress a few compile warnings

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/lineedit.c b/libbb/lineedit.c
index de236de..e40a720 100644
--- a/libbb/lineedit.c
+++ b/libbb/lineedit.c
@@ -99,7 +99,7 @@
 # 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)