Fix SpotBugs AT_STALE_THREAD_WRITE_OF_PRIMITIVE in ProxyInputStream

[ERROR] Medium: Shared primitive variable "closed" in one thread may not
yield the value of the most recent write from another thread
[org.apache.commons.io.input.ProxyInputStream] At
ProxyInputStream.java:[line 233] AT_STALE_THREAD_WRITE_OF_PRIMITIVE

Author: garydgregory

src/changes/changes.xml

@@ -64,6 +64,7 @@ The <action> type attribute can be add,update,fix,remove.
       <action dev="ggregory" type="fix"                due-to="Gary Gregory">Fix SpotBugs [ERROR] Medium: Shared primitive variable "atEos" in one thread may not yield the value of the most recent write from another thread [org.apache.commons.io.input.UnixLineEndingInputStream] At UnixLineEndingInputStream.java:[line 120] AT_STALE_THREAD_WRITE_OF_PRIMITIVE.</action>
       <action dev="ggregory" type="fix"                due-to="Gary Gregory">Fix SpotBugs [ERROR] Medium: Shared primitive variable "atSlashCr" in one thread may not yield the value of the most recent write from another thread [org.apache.commons.io.input.UnixLineEndingInputStream] At UnixLineEndingInputStream.java:[line 124] AT_STALE_THREAD_WRITE_OF_PRIMITIVE.</action>
       <action dev="ggregory" type="fix"                due-to="Gary Gregory">Fix SpotBugs [ERROR] Medium: Shared primitive variable "atSlashLf" in one thread may not yield the value of the most recent write from another thread [org.apache.commons.io.input.UnixLineEndingInputStream] At UnixLineEndingInputStream.java:[line 125] AT_STALE_THREAD_WRITE_OF_PRIMITIVE.</action>
+      <action dev="ggregory" type="fix"                due-to="Gary Gregory">Fix SpotBugs [ERROR] Medium: Shared primitive variable "closed" in one thread may not yield the value of the most recent write from another thread [org.apache.commons.io.input.ProxyInputStream] At ProxyInputStream.java:[line 233] AT_STALE_THREAD_WRITE_OF_PRIMITIVE.</action>
       <!-- ADD -->
       <action dev="ggregory" type="add" issue="IO-875" due-to="Pierre Baumard, Gary Gregory">Add and use org.apache.commons.io.file.CountingPathVisitor.accept(Path, BasicFileAttributes) #743.</action> 
       <!-- UPDATE -->

src/main/java/org/apache/commons/io/input/ProxyInputStream.java

@@ -100,7 +100,7 @@ public B setAfterRead(final IOIntConsumer afterRead) {
     /**
      * Tracks whether {@link #close()} has been called or not.
      */
-    private boolean closed;
+    private volatile boolean closed;
 
     /**
      * Handles exceptions.