ensure we don't pass a negative int to ctypes functions

Some implementations, e.g. cygwin, use a table lookup that can cast
a char to a negative array offset.

1 files changed, 3 insertions, 3 deletions

diff --git a/apps/nc/compat/readpassphrase.c b/apps/nc/compat/readpassphrase.c
index 0d04134..f3aa248 100644
--- a/apps/nc/compat/readpassphrase.c
+++ b/apps/nc/compat/readpassphrase.c
@@ -141,11 +141,11 @@ restart:
                         if (p < end) {
                                 if ((flags & RPP_SEVENBIT))
                                         ch &= 0x7f;
-                                if (isalpha(ch)) {
+                                if (isalpha((unsigned char)ch)) {
                                         if ((flags & RPP_FORCELOWER))
-                                                ch = (char)tolower(ch);
+                                                ch = (char)tolower((unsigned char)ch);
                                         if ((flags & RPP_FORCEUPPER))
-                                                ch = (char)toupper(ch);
+                                                ch = (char)toupper((unsigned char)ch);
                                 }
                                 *p++ = ch;
                         }