Clarify |RSA_verify_raw| error handling & cleanup.

Use the common pattern of returning early instead of |goto err;| when
there's no cleanup to do yet. Also, move the error checking of
|BN_CTX_get| failure closer to the the calls to |BN_CTX_get|. Avoid
calling |OPENSSL_cleanse| on public data. Clarify when/why |buf| is not
freed.

Change-Id: I9df833db7eb7041c5af9349c461297372b988f98
Reviewed-on: https://boringssl-review.googlesource.com/7464
Reviewed-by: David Benjamin <davidben@google.com>

crypto/rsa/rsa_impl.c

diff --git a/crypto/rsa/rsa_impl.c b/crypto/rsa/rsa_impl.c
index fecb911..35b2623 100644
--- a/crypto/rsa/rsa_impl.c
+++ b/crypto/rsa/rsa_impl.c
@@ -435,10 +435,7 @@
 
   const unsigned rsa_size = RSA_size(rsa);
   BIGNUM *f, *result;
-  int ret = 0;
   int r = -1;
-  uint8_t *buf = NULL;
-  BN_CTX *ctx = NULL;
 
   if (max_out < rsa_size) {
     OPENSSL_PUT_ERROR(RSA, RSA_R_OUTPUT_BUFFER_TOO_SMALL);
@@ -454,14 +451,22 @@
     return 0;
   }
 
-  ctx = BN_CTX_new();
+  BN_CTX *ctx = BN_CTX_new();
   if (ctx == NULL) {
-    goto err;
+    return 0;
   }
 
+  int ret = 0;
+  uint8_t *buf = NULL;
+
   BN_CTX_start(ctx);
   f = BN_CTX_get(ctx);
   result = BN_CTX_get(ctx);
+  if (f == NULL || result == NULL) {
+    OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
+    goto err;
+  }
+
   if (padding == RSA_NO_PADDING) {
     buf = out;
   } else {
@@ -472,10 +477,6 @@
       goto err;
     }
   }
-  if (!f || !result) {
-    OPENSSL_PUT_ERROR(RSA, ERR_R_MALLOC_FAILURE);
-    goto err;
-  }
 
   if (BN_bin2bn(in, in_len, f) == NULL) {
     goto err;
@@ -516,12 +517,9 @@
   }
 
 err:
-  if (ctx != NULL) {
-    BN_CTX_end(ctx);
-    BN_CTX_free(ctx);
-  }
-  if (padding != RSA_NO_PADDING && buf != NULL) {
-    OPENSSL_cleanse(buf, rsa_size);
+  BN_CTX_end(ctx);
+  BN_CTX_free(ctx);
+  if (buf != out) {
     OPENSSL_free(buf);
   }
   return ret;