[fix] sync SSLContext#setup as it could be shared (#302)

follow-up on previous commit: b612897

Author: kares

src/main/java/org/jruby/ext/openssl/SSLContext.java

@@ -315,7 +315,7 @@ public SSLContext(Ruby runtime, RubyClass type) {
     //private int sessionCacheMode; // 2 default on MRI
     private int sessionCacheSize; // 20480
 
-    private InternalContext internalContext;
+    private volatile InternalContext internalContext;
 
     @JRubyMethod(required = 0, optional = 1, visibility = Visibility.PRIVATE)
     public IRubyObject initialize(IRubyObject[] args) {
@@ -334,7 +334,7 @@ public IRubyObject initialize_copy(IRubyObject original) {
     final SSLContext initializeImpl() { return this; }
 
     @JRubyMethod
-    public IRubyObject setup(final ThreadContext context) {
+    public synchronized IRubyObject setup(final ThreadContext context) {
         final Ruby runtime = context.runtime;
 
         if ( isFrozen() ) return runtime.getNil();