fix 'signed char' to 'int' conversion [cert-str34-c]

Fortunately here we don't need to convert to unsigned char then int,
we can do the only comparisons we need staying in the char domain.

src/dsk.c

@@ -2502,8 +2502,8 @@ int true_name(register char *path)
 #ifdef DOS
   char drive[1];
 #endif
-  register char *sp, *cp;
+  register char c, *sp, *cp;
-  register int type, c;
+  register int type;
 
   if (strcmp(path, "/") == 0) return (-1);