safer code, test for return of fcntl

TraceScreen.c

@@ -87,9 +87,11 @@ void TraceScreen_run(TraceScreen* this) {
    if (child == -1) return;
    if (child == 0) {
       dup2(fdpair[1], STDERR_FILENO);
-      fcntl(fdpair[1], F_SETFL, O_NONBLOCK);
-      sprintf(buffer, "%d", this->process->pid);
-      execlp("strace", "strace", "-p", buffer, NULL);
+      int ok = fcntl(fdpair[1], F_SETFL, O_NONBLOCK);
+      if (ok != -1) {
+         sprintf(buffer, "%d", this->process->pid);
+         execlp("strace", "strace", "-p", buffer, NULL);
+      }
       const char* message = "Could not execute 'strace'. Please make sure it is available in your $PATH.";
       write(fdpair[1], message, strlen(message));
       exit(1);