add buffer_tosa check to marshal.c

failed attempt to improve scan_ulongn, results in essentially same code
but the C code looks slighly better so I'm keeping it

scan/scan_ulongn.c

@@ -14,10 +14,8 @@ size_t scan_ulongn(const char* src,size_t n,unsigned long int* dest) {
      * so instead of *10 we do (l<<3) (i.e. *8) + (l<<1) (i.e. *2)
      * and check for overflow on all the intermediate steps */
     n=l<<3; if ((n>>3)!=l) break;
-    if (n+(l<<1) < n) break;
-    n+=l<<1;
-    if (n+c < n) break;
-    l=n+c;
+    if (n+(l<<1)+c < n) break;
+    l=n+(l<<1)+c;
     ++tmp;
   }
   if (tmp-src) *dest=l;

test/marshal.c

@@ -398,12 +398,22 @@ int main() {
   {
     buffer b;
     stralloc s;
+    size_t i;
     buffer_fromsa(&b,&sa);
     stralloc_init(&s);
     assert(buffer_getline_sa(&b,&s)==1 && stralloc_equals(&s,"foo\n"));
     assert(buffer_getnewline_sa(&b,&s)==1 && stralloc_equals(&s,"bar\r\n"));
     assert(buffer_getnewline_sa(&b,&s)==0 && stralloc_equals(&s,"baz"));
     buffer_close(&b);
+
+    stralloc_free(&s);
+    buffer_tosa(&b,&s);
+    for (i=0; i<100; ++i)
+      buffer_puts(&b,"foo bar baz!\n");
+    buffer_flush(&b);
+    assert(s.len==100*sizeof("foo bar baz!"));
+    for (i=0; i<100; ++i)
+      assert(byte_equal(s.s+i*sizeof("foo bar baz!"),sizeof("foo bar baz!"),"foo bar baz!\n"));
   }
 
   return 0;