Tolerate a null BN_CTX in BN_primality_test.

This used to work, but I broke it on accident in the recent rewrite.

Change-Id: I06ab5e06eb0c0a6b67ecc97919654e386f3c2198
Reviewed-on: https://boringssl-review.googlesource.com/26984
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/fipsmodule/bn/bn_test.cc

@@ -2031,6 +2031,13 @@ TEST_F(BNTest, PrimeChecking) {
         &result_3, p.get(), BN_prime_checks, ctx(), nullptr /* callback */));
     EXPECT_EQ(bn_composite, result_3);
   }
+
+  // BN_primality_test works with null |BN_CTX|.
+  ASSERT_TRUE(BN_set_word(p.get(), 5));
+  ASSERT_TRUE(BN_primality_test(
+      &is_probably_prime_1, p.get(), BN_prime_checks, nullptr /* ctx */,
+      false /* do_trial_division */, nullptr /* callback */));
+  EXPECT_EQ(1, is_probably_prime_1);
 }
 
 TEST_F(BNTest, NumBitsWord) {

crypto/fipsmodule/bn/prime.c

@@ -647,6 +647,15 @@ int BN_primality_test(int *is_probably_prime, const BIGNUM *w,
     iterations = BN_prime_checks_for_size(BN_num_bits(w));
   }
 
+  BN_CTX *new_ctx = NULL;
+  if (ctx == NULL) {
+    new_ctx = BN_CTX_new();
+    if (new_ctx == NULL) {
+      return 0;
+    }
+    ctx = new_ctx;
+  }
+
   // See C.3.1 from FIPS 186-4.
   int ret = 0;
   BN_MONT_CTX *mont = NULL;
@@ -792,6 +801,7 @@ int BN_primality_test(int *is_probably_prime, const BIGNUM *w,
 err:
   BN_MONT_CTX_free(mont);
   BN_CTX_end(ctx);
+  BN_CTX_free(new_ctx);
   return ret;
 }