Don't call base64_ascii_to_bin twice on each byte.

With the constant-time base64 decode, base64_ascii_to_bin is a bit more
expensive. This check is redundant with the one in base64_decode_quad,
though it does mean syntax error reporting will be slightly deferred by
four bytes.

Change-Id: I71f23ea23feba2ee5b41df79ce09026fb56996d3
Reviewed-on: https://boringssl-review.googlesource.com/16444
Reviewed-by: Adam Langley <agl@google.com>

crypto/base64/base64.c

diff --git a/crypto/base64/base64.c b/crypto/base64/base64.c
index a47cb15..1ddf094 100644
--- a/crypto/base64/base64.c
+++ b/crypto/base64/base64.c
@@ -327,7 +327,7 @@
         continue;
     }
 
-    if (base64_ascii_to_bin(c) == 0xff || ctx->eof_seen) {
+    if (ctx->eof_seen) {
       ctx->error_encountered = 1;
       return -1;
     }