[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_io.cpp b/adb/adb_io.cpp
index 38e3116..6cc274b 100644
--- a/adb/adb_io.cpp
+++ b/adb/adb_io.cpp
@@ -49,7 +49,7 @@
     }
     buf[4] = 0;
 
-    unsigned long len = strtoul(buf, 0, 16);
+    unsigned long len = strtoul(buf, nullptr, 16);
     s->resize(len, '\0');
     if (!ReadFdExactly(fd, &(*s)[0], len)) {
         *error = perror_str("protocol fault (couldn't read status message)");