Add OPENSSL_zalloc

OpenSSL added a similar helper function. It's very, very common for us
to malloc something an then zero it. This saves some effort. Also
replace some more malloc + memcpy pairs with memdup.

Change-Id: I1e765c8774a0d15742827c39a1f16df9748ef247
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/63345
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Auto-Submit: David Benjamin <davidben@google.com>

ssl/d1_both.cc

diff --git a/ssl/d1_both.cc b/ssl/d1_both.cc
index 55c92fa..b910b96 100644
--- a/ssl/d1_both.cc
+++ b/ssl/d1_both.cc
@@ -184,11 +184,10 @@
       return nullptr;
     }
     size_t bitmask_len = (msg_hdr->msg_len + 7) / 8;
-    frag->reassembly = (uint8_t *)OPENSSL_malloc(bitmask_len);
+    frag->reassembly = (uint8_t *)OPENSSL_zalloc(bitmask_len);
     if (frag->reassembly == NULL) {
       return nullptr;
     }
-    OPENSSL_memset(frag->reassembly, 0, bitmask_len);
   }
 
   return frag;