Tidy up primality code.

We BN_cmp with 1 at the top, so the absolute value code never runs.
This simplifies the BN_CTX business considerably. Also add a test for
negative prime numbers.

Change-Id: I500a56bc285c2f75576947cfb518e75c9e6861ce
Reviewed-on: https://boringssl-review.googlesource.com/15367
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/bn/bn_test.cc

diff --git a/crypto/bn/bn_test.cc b/crypto/bn/bn_test.cc
index fc48339..2d92276 100644
--- a/crypto/bn/bn_test.cc
+++ b/crypto/bn/bn_test.cc
@@ -2739,6 +2739,26 @@
     }
   }
 
+  // Negative numbers are not prime.
+  if (!BN_set_word(p.get(), 7)) {
+    return false;
+  }
+  BN_set_negative(p.get(), 1);
+  if (!BN_primality_test(&is_probably_prime_1, p.get(), BN_prime_checks, ctx,
+                         false /* do_trial_division */,
+                         nullptr /* callback */) ||
+      is_probably_prime_1 != 0 ||
+      !BN_primality_test(&is_probably_prime_2, p.get(), BN_prime_checks, ctx,
+                         true /* do_trial_division */,
+                         nullptr /* callback */) ||
+      is_probably_prime_2 != 0) {
+    fprintf(stderr,
+            "TestPrimeChecking failed for -7 (is_prime: 0 vs %d without "
+            "trial division vs %d with it)\n",
+            is_probably_prime_1, is_probably_prime_2);
+    return false;
+  }
+
   return true;
 }