Avoid errors caused by fseek after ungetc on read-only files.

The error could occur because fseek calls fflush followed by ftell. fflush would reset the file position as if the characters in the putback buffer were removed, but ftell would still see them and try to adjust for them (in the case of a read-only file). This could result in trying to seek before the beginning of the file, producing an error.

Here is a program that was affected:

#include <stdio.h>
int main(void) {
        FILE *f = fopen("somefile","r");
        if (!f) return 0;
        fgetc(f);
        ungetc('X', f);
        fseek(f, 0, SEEK_CUR);
        if (ferror(f)) puts("error encountered");
}

stdio.asm

@@ -4623,6 +4623,11 @@ ib2      ldy   #FILE_cnt+2
          dey
          txa
          sta   [stream],Y
+         ldy   #FILE_pbk                nothing in the putback buffer
+         lda   #$FFFF
+         sta   [stream],Y
+         ldy   #FILE_pbk+2
+         sta   [stream],Y
 
          creturn
          end