[9fans] fix for bwk awk problem with split

[Aharon Robbins]

This makes it work correctly for me on Linux. Not sure about 9awk.

I have corresponded with BWK about this, including sending him this
proposed patch.

Arnold
----------------
--- run.c.orig  2012-12-10 22:50:40.403115427 +0200
+++ run.c       2012-12-10 22:50:15.450991568 +0200
@@ -1216,6 +1216,7 @@
        int n, tempstat, arg3type;
 
        y = execute(a[0]);      /* source string */
+       y->tval |= DONTFREE;
        s = getsval(y);
        arg3type = ptoi(a[3]);
        if (a[2] == 0)          /* fs string */
@@ -1335,6 +1336,7 @@
                }
        }
        tempfree(ap);
+       y->tval &= ~DONTFREE;
        tempfree(y);
        if (a[2] != 0 && arg3type == STRING) {
                tempfree(x);