Always check available() in InputPumper to avoid burning CPU (#4095)

Attempts to fix #4092

Instead of checking `src.available() == 0`, we check for
`src.read(buffer) == 0`, which should provide a more accurate measure

main/client/src/mill/main/client/InputPumper.java

@@ -36,19 +36,17 @@ public void run() {
     byte[] buffer = new byte[1024];
     try {
       while (running) {
-        if (!runningCheck.getAsBoolean()) {
-          running = false;
-        } else if (checkAvailable && src.available() == 0) Thread.sleep(1);
+        if (!runningCheck.getAsBoolean()) running = false;
         else {
           int n;
           try {
             n = src.read(buffer);
           } catch (Exception e) {
             n = -1;
           }
-          if (n == -1) {
-            running = false;
-          } else {
+          if (n == -1) running = false;
+          else if (n == 0) Thread.sleep(1);
+          else {
             try {
               dest.write(buffer, 0, n);
               dest.flush();