patch 8.2.4418: crash when using special multi-byte character

Problem:    Crash when using special multi-byte character.
Solution:   Don't use isalpha() for an arbitrary character.
diff --git a/src/proto/charset.pro b/src/proto/charset.pro
index ee5370b..df09710 100644
--- a/src/proto/charset.pro
+++ b/src/proto/charset.pro
@@ -50,6 +50,7 @@
 int vim_isbdigit(int c);
 int vim_islower(int c);
 int vim_isupper(int c);
+int vim_isalpha(int c);
 int vim_toupper(int c);
 int vim_tolower(int c);
 char_u *skiptowhite(char_u *p);
@@ -63,5 +64,4 @@
 int rem_backslash(char_u *str);
 void backslash_halve(char_u *p);
 char_u *backslash_halve_save(char_u *p);
-void ebcdic2ascii(char_u *buffer, int len);
 /* vim: set ft=c : */