updater: Don't zero out CommandParameters with memset(3).

[1] switched a few things to android::base::unique_fd including
CommandParameters.fd. However, we were using memset(3) to zero out the
struct, which effectively assigned unique_fd(0) to fd. When it called
fd.reset(), file descriptor 0 was unintentionally closed. When FD 0 was
later reassigned via open(2), it led to lseek(2) errors: "Bad file
descriptor".

This CL switches to using braced-init (i.e. '= {}') instead, so that the
default constructor unique_fd(-1) would be called.

[1]: commit bcabd09293

Bug: 28391985
Change-Id: If1f99932b15552714c399e65c8b80550344b758a

updater/blockimg.cpp

@@ -1318,8 +1318,7 @@ static unsigned int HashString(const char *s) {
 
 static Value* PerformBlockImageUpdate(const char* name, State* state, int /* argc */, Expr* argv[],
         const Command* commands, size_t cmdcount, bool dryrun) {
-    CommandParameters params;
-    memset(&params, 0, sizeof(params));
+    CommandParameters params = {};
     params.canwrite = !dryrun;
 
     fprintf(stderr, "performing %s\n", dryrun ? "verification" : "update");