Fix check of the pointer returned by BN_CTX_get

The check of `r` instead of `rr` was introduced in change
I298400b988e3bd108d01d6a7c8a5b262ddf81feb.

Change-Id: I4376a81c65856f6457b0a11276176bf35e9c647d
Reviewed-on: https://boringssl-review.googlesource.com/31844
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/fipsmodule/bn/mul.c

diff --git a/crypto/fipsmodule/bn/mul.c b/crypto/fipsmodule/bn/mul.c
index bd9393e..a1582a2 100644
--- a/crypto/fipsmodule/bn/mul.c
+++ b/crypto/fipsmodule/bn/mul.c
@@ -559,7 +559,7 @@
   BN_CTX_start(ctx);
   if (r == a || r == b) {
     rr = BN_CTX_get(ctx);
-    if (r == NULL) {
+    if (rr == NULL) {
       goto err;
     }
   } else {