[adb] Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: Ia8c4deacafed2f3b7dbc3d4c3c77c6c632e3de81
diff --git a/adb/adb.cpp b/adb/adb.cpp
index f8a54c6..20d0db5 100644
--- a/adb/adb.cpp
+++ b/adb/adb.cpp
@@ -415,7 +415,7 @@
         if (t->online && p->msg.arg0 != 0 && p->msg.arg1 != 0) {
             asocket* s = find_local_socket(p->msg.arg1, 0);
             if (s) {
-                if(s->peer == 0) {
+                if(s->peer == nullptr) {
                     /* On first READY message, create the connection. */
                     s->peer = create_remote_socket(p->msg.arg0, t);
                     s->peer->peer = s;