Make BN_clear_free a wrapper around BN_free.

We clear all heap memory on free now, thus the difference between these
functions is quite small. There are some differences though:

Firstly, BN_clear_free will attempt to zero out static limb data.  But
static data is probably read-only and thus trying to zero it will crash.

Secondly it will try to zero out the BIGNUM structure itself. But either
it's on the heap, and will be zeroed anyway, or else it's on the stack,
and we don't try and clear the stack in general because the compiler is
duplicating bits of it at will anyway.

Change-Id: I8a07385a102cfd308b555432942225c25eb7c12d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/45084
Reviewed-by: David Benjamin <davidben@google.com>

crypto/fipsmodule/bn/bn.c

@@ -101,26 +101,7 @@ void BN_free(BIGNUM *bn) {
 }
 
 void BN_clear_free(BIGNUM *bn) {
-  char should_free;
-
-  if (bn == NULL) {
-    return;
-  }
-
-  if (bn->d != NULL) {
-    if ((bn->flags & BN_FLG_STATIC_DATA) == 0) {
-      OPENSSL_free(bn->d);
-    } else {
-      OPENSSL_cleanse(bn->d, bn->dmax * sizeof(bn->d[0]));
-    }
-  }
-
-  should_free = (bn->flags & BN_FLG_MALLOCED) != 0;
-  if (should_free) {
-    OPENSSL_free(bn);
-  } else {
-    OPENSSL_cleanse(bn, sizeof(BIGNUM));
-  }
+  BN_free(bn);
 }
 
 BIGNUM *BN_dup(const BIGNUM *src) {