Unify mbtowc() invocation and error handling

An abstraction worth performing since we can unify the internal state
reset and there's currently no need to distinguish between the two
scenarios where mbtowc() returns -1 or 0.

pick.c

@@ -92,6 +92,7 @@ static int			 tty_putc(int);
 static void			 tty_restore(void);
 static void			 tty_size(void);
 static __dead void		 usage(void);
+static int			 xmbtowc(wchar_t *, const char *);
 
 static struct termios		 original_attributes;
 static struct {
@@ -625,13 +626,8 @@ strcasechr(const char *s1, const char *s2)
 	size_t	i;
 	int	in_esc_seq, nbytes;
 
-	switch (mbtowc(&wc2, s2, MB_CUR_MAX)) {
-	case -1:
-		mbtowc(NULL, NULL, MB_CUR_MAX);
-		/* FALLTHROUGH */
-	case 0:
+	if (xmbtowc(&wc2, s2) == 0)
 		return NULL;
-	}
 
 	in_esc_seq = 0;
 	for (i = 0; s1[i] != '\0';) {
@@ -642,16 +638,13 @@ strcasechr(const char *s1, const char *s2)
 				in_esc_seq = 0;
 		} else if (i > 0 && s1[i - 1] == '\033' && s1[i] == '[') {
 			in_esc_seq = 1;
-		} else if ((nbytes = mbtowc(&wc1, &s1[i], MB_CUR_MAX)) == -1) {
-			mbtowc(NULL, NULL, MB_CUR_MAX);
+		} else if ((nbytes = xmbtowc(&wc1, &s1[i])) == 0) {
+			nbytes = 1;
 		} else if (wcsncasecmp(&wc1, &wc2, 1) == 0) {
 			return &s1[i];
 		}
 
-		if (nbytes > 0)
-			i += nbytes;
-		else
-			i++;
+		i += nbytes;
 	}
 
 	return NULL;
@@ -816,12 +809,7 @@ print_line(const char *str, size_t len, int standout,
 			continue;
 		}
 
-		/*
-		 * Due to the explicit NUL-check above, the case where
-		 * mbtowc(3) returns 0 is not handled here.
-		 */
-		if ((nbytes = mbtowc(&wc, &str[i], MB_CUR_MAX)) == -1) {
-			mbtowc(NULL, NULL, MB_CUR_MAX);
+		if ((nbytes = xmbtowc(&wc, &str[i])) == 0) {
 			i++;
 			continue;
 		}
@@ -1072,3 +1060,17 @@ isword(const char *s)
 
 	return iswalnum(wc) || wc == L'_';
 }
+
+int
+xmbtowc(wchar_t *wc, const char *s)
+{
+	int	n;
+
+	n = mbtowc(wc, s, MB_CUR_MAX);
+	if (n == -1) {
+		mbtowc(NULL, NULL, MB_CUR_MAX);
+		return 0;
+	}
+
+	return n;
+}