A cleanup for MDEV-17088 Provide tools to encode/decode mysql-encoded file system names

tests mariadb-conv-utf8 and mariadb-conv-cp932 failed on PPC,
because "char" is "unsigned char" on PPC.

Adding a cast from "char" to "signed char" in the two affected places.

client/mariadb-conv.cc

@@ -188,7 +188,7 @@ class Delimiter
   }
   bool is_delimiter(char ch) const
   {
-    return ch < 0 ? false : m_delimiter[(uint32) ch];
+    return (signed char) ch < 0 ? false : m_delimiter[(uint32) ch];
   }
 public:
   Delimiter()
@@ -205,7 +205,7 @@ class Delimiter
     m_has_delimiter_cached= false;
     for ( ; *str; str++)
     {
-      if (*str < 0)
+      if ((signed char) *str < 0)
         return true;
       if (*str == '\\')
       {