testbn: add a missing initialisation in argument setup.

The code that parses hexadecimal test arguments out of test lines
writes them into a buffer in binary form, and sets ptrs[i] to be the
starting point of each argument. The idea is that ptrs[i+1]-ptrs[i] is
the length of each argument - but for that to apply to the _final_
argument, we need to have set one final element in ptrs[], which I
forgot to do.

testbn.c

@@ -90,6 +90,9 @@ int main(int argc, char **argv)
             }
         }
 
+        if (ptrnum < lenof(ptrs))
+            ptrs[ptrnum] = q;
+
         if (!strcmp(buf, "mul")) {
             Bignum a, b, c, p;