examples: don't promote unchecked function calls

* etc/bench.pl.in, examples/c/bistromathic/parse.y,
* examples/c/calc/calc.y, examples/c/pushcalc/calc.y: Check scanf's
return value.
* doc/bison.texi: Likewise, but only for the second example, to avoid
cluttering the very simple case.

examples/c/calc/calc.y

@@ -1,4 +1,5 @@
 %code top {
+  #include <assert.h>
   #include <ctype.h>  /* isdigit. */
   #include <stdio.h>  /* For printf, etc. */
   #include <string.h> /* strcmp. */
@@ -73,7 +74,8 @@ yylex (void)
   if (c == '.' || isdigit (c))
     {
       ungetc (c, stdin);
-      scanf ("%lf", &yylval.NUM);
+      int n = scanf ("%lf", &yylval.NUM);
+      assert (n == 1);
       return NUM;
     }