Adding missing BN_CTX_start/BN_CTX_end in ec_key

Change-Id: Icfa6a0bc36b808e2e6ea8b36a0fc49b3c4943b07
Reviewed-on: https://boringssl-review.googlesource.com/7254
Reviewed-by: David Benjamin <davidben@google.com>
diff --git a/crypto/ec/ec_key.c b/crypto/ec/ec_key.c
index 480ed5f..fee71fe 100644
--- a/crypto/ec/ec_key.c
+++ b/crypto/ec/ec_key.c
@@ -365,10 +365,15 @@
     return 0;
   }
   ctx = BN_CTX_new();
+
+  if (ctx == NULL) {
+    return 0;
+  }
+
+  BN_CTX_start(ctx);
   point = EC_POINT_new(key->group);
 
-  if (ctx == NULL ||
-      point == NULL) {
+  if (point == NULL) {
     goto err;
   }
 
@@ -402,6 +407,7 @@
   ok = 1;
 
 err:
+  BN_CTX_end(ctx);
   BN_CTX_free(ctx);
   EC_POINT_free(point);
   return ok;