Cleanup on randomization

tsan reported that rand() is not thread safe, so create
a thread safe wrapper, use rand_r() if available.
Consolidate the function, initialization and cleanup in
dict.c and make sure it is initialized in xmlInitParser()

parser.c

@@ -14178,6 +14178,7 @@ xmlInitParser(void) {
 	    (xmlGenericError == NULL))
 	    initGenericErrorDefaultFunc(NULL);
 	xmlInitMemory();
+        xmlInitializeDict();
 	xmlInitCharEncodingHandlers();
 	xmlDefaultSAXHandlerInit();
 	xmlRegisterDefaultInputCallbacks();