Fix a memory leak

If create_service_thread fails, we'll leak `arg`.

This fixes a static analyzer complaint:
system/core/adb/services.cpp:298:13: warning: Potential leak of memory
pointed to by 'arg'

Bug: None
Test: Reran the static analyzer. No more complaints about this leak.
Change-Id: I5aec7fd78f2cc775b650501b02bdf0039d1647ca

adb/services.cpp

@@ -296,6 +296,7 @@ int service_to_fd(const char* name, const atransport* transport) {
         void* arg = strdup(name + 7);
         if (arg == NULL) return -1;
         ret = create_service_thread("reboot", reboot_service, arg);
+        if (ret < 0) free(arg);
     } else if(!strncmp(name, "root:", 5)) {
         ret = create_service_thread("root", restart_root_service, nullptr);
     } else if(!strncmp(name, "unroot:", 7)) {