Fix another bug reported by Wipro: "...at line 89 in function xlungetc , file xlio.c

'fptr', can take a NULL value. It is being derefed directly or through its aliases at: '(((fptr- >n_info).n_xstream).xs_savech)'"

lib-src/libnyquist/nyquist/xlisp/xlio.c

@@ -72,7 +72,7 @@ void xlungetc(LVAL fptr, int ch)
     
     /* check for ungetc from nil */
     if (fptr == NIL)
-        ;
+        ; //ANSWER-ME: Return? "else if"?
         
     /* otherwise, check for ungetc to a stream */
     if (ustreamp(fptr)) {
@@ -85,7 +85,7 @@ void xlungetc(LVAL fptr, int ch)
     }
     
     /* otherwise, it must be a file */
-    else
+    else if (fptr)
         setsavech(fptr,ch);
 }