As written, getint treats a + or – not followed by a digit as a valid representation of zero. Fix it to push such a character back on the input.

#include <stdio.h>
#include <ctype.h>

int getch(void);
void ungetch(int);

/* getint: get next integer from input into *pn      */
int getint(int *pn){
     int c, d, sign;

     while(isspace(c =getch()))  /* skip white space   */
              ;
     if(!isdigit(c) && c != EOF && c != '+' && c != '-') {
          ungetch(c);     /* it's not a number */
          return 0;
     }
     sign = (c == '-') ? -1 : 1;
     if (c == '+' || c == '-') {
         d = c ;       /* remember sign char */
         if(!isdigit(c = getch())) {
            if (c != EOF)
                 ungetch(c);     /* push back non-digit */
            ungetch(d);         /* push back sign char  */
            return d;
         }
     }
     for(*pn = 0; isdigit(c); c =getch())
        *pn = 10 * *pn + (c - '0');
     * pn += sign;
     if (c != EOF)
           ungetch(c);
     return c;
}