Don't read past the end of the string in BUF_strndup.

BUF_strlcpy still assumes |src| is a NUL-terminated string and will call strlen
on it to determine the actual length. BUF_strndup's input need not be
NUL-terminated.

Change-Id: I9ca95e92533d12f1b0283412249bda4f8cf92433
Reviewed-on: https://boringssl-review.googlesource.com/1997
Reviewed-by: Adam Langley <agl@google.com>

crypto/buf/buf.c

@@ -187,7 +187,8 @@ char *BUF_strndup(const char *buf, size_t size) {
     return NULL;
   }
 
-  BUF_strlcpy(ret, buf, alloc_size);
+  memcpy(ret, buf, size);
+  ret[size] = '\0';
   return ret;
 }