Check maximum properly in crypto_rand_int()

George Kadianakis notes that if you give crypto_rand_int() a value
above INT_MAX, it can return a negative number, which is not what
the documentation would imply.

The simple solution is to assert that the input is in [1,INT_MAX+1].
If in the future we need a random-value function that can return
values up to UINT_MAX, we can add one.

Fixes bug 3306; bugfix on 0.2.2pre14.

changes/bug3306

@@ -0,0 +1,5 @@
+  o Minor bugfixes:
+    - Make our crypto_rand_int() function check the value of its input
+      correctly. Previously, it accepted values up to UINT_MAX, but
+      could return a negative number if given a value above INT_MAX+1.
+      Found by George Kadianakis. Fixes bug 3306; bugfix on 0.2.2pre14.

src/common/crypto.c

@@ -2145,13 +2145,14 @@ crypto_rand(char *to, size_t n)
 }
 
 /** Return a pseudorandom integer, chosen uniformly from the values
- * between 0 and <b>max</b>-1. */
+ * between 0 and <b>max</b>-1 inclusive.  <b>max</b> must be between 1 and
+ * INT_MAX+1, inclusive. */
 int
 crypto_rand_int(unsigned int max)
 {
   unsigned int val;
   unsigned int cutoff;
-  tor_assert(max < UINT_MAX);
+  tor_assert(max <= ((unsigned int)INT_MAX)+1);
   tor_assert(max > 0); /* don't div by 0 */
 
   /* We ignore any values that are >= 'cutoff,' to avoid biasing the