Fix a bad parameter in `PEM_read_bio_inner`'s base64 decoding.

This would have been an OOB write if `EVP_DecodeFinal` ever wrote anything. But it doesn't write anything, so this CL is a no-op.

Change-Id: I4e56ff22b04ece37899ee330a2cf41dc6a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/97807
Commit-Queue: Rudolf Polzer <rpolzer@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Rudolf Polzer <rpolzer@google.com>

crypto/pem/pem_lib.cc

diff --git a/crypto/pem/pem_lib.cc b/crypto/pem/pem_lib.cc
index 0e4d75b..83266f4 100644
--- a/crypto/pem/pem_lib.cc
+++ b/crypto/pem/pem_lib.cc
@@ -645,7 +645,8 @@
     return 0;
   }
   int k;
-  status = EVP_DecodeFinal(&ctx, (unsigned char *)&(dataB->data[bl]), &k);
+  status = EVP_DecodeFinal(&ctx,
+                           (unsigned char *)&(dataB->data[decoded_length]), &k);
   if (status < 0) {
     OPENSSL_PUT_ERROR(PEM, PEM_R_BAD_BASE64_DECODE);
     return 0;