Fix SHA-384 and -512 in NO_ASM mode.

This transcription bug comes from the start of BoringSSL and, as you can
imagine, was a complete delight to track down.

Change-Id: I3051934195098a1d3bf893b154389ec7f14d3609
Reviewed-on: https://boringssl-review.googlesource.com/3740
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/sha.h

@@ -224,7 +224,7 @@ struct sha512_state_st {
   uint64_t Nl, Nh;
   union {
     uint64_t d[16];
-    uint8_t p[64];
+    uint8_t p[128];
   } u;
   unsigned int num, md_len;
 };