Fix use of unitialized cbb on failure case.

This made fido2's fuzzer angry: https://buganizer.corp.google.com/issues/271220905

Change-Id: Ib1b909be10f230df2daea3942f35cba0a81dcedb
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/57765
Commit-Queue: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Auto-Submit: Bob Beck <bbe@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/asn1/a_mbstr.c

diff --git a/crypto/asn1/a_mbstr.c b/crypto/asn1/a_mbstr.c
index 80cfac4..85a7b98 100644
--- a/crypto/asn1/a_mbstr.c
+++ b/crypto/asn1/a_mbstr.c
@@ -222,6 +222,8 @@
     }
   }
 
+  CBB cbb;
+  CBB_zero(&cbb);
   // If both the same type just copy across
   if (inform == outform) {
     if (!ASN1_STRING_set(dest, in, len)) {
@@ -231,8 +233,6 @@
     *out = dest;
     return str_type;
   }
-
-  CBB cbb;
   if (!CBB_init(&cbb, size_estimate + 1)) {
     goto err;
   }