Fix spurious joblog message removal code

This was added in 3c28f3b, but it didn't actually work due to an
inverted if check. Fix the check and add some more documentation to the
implementation.

IBM_DB/ibm_db/ibm_db.c

@@ -11339,7 +11339,9 @@ static int set_joblog_message_marker() {
     );
 
     if (rc == -1) {
-        return 0x40404040;
+        // When receiving 0 is a special value for *NEXT/*PREV, which we don't
+        // use and it's also a nice boolean false value
+        return 0;
     }
 
     return key;
@@ -11368,10 +11370,18 @@ static void clear_joblog_messages(int key) {
     int rc;
 
     // Ignore invalid message key
-    if (key) return;
+    if (!key) return;
 
+    // Retrieve all the messages from the most recent up to and including the
+    // marker message
     while (1) {
-        outkey = 0x40404040;
+        // "If you are not receiving messages by key, use blanks for this
+        // parameter."
+        outkey = 0x40404040; // '    ' in EBCDIC
+
+        // Receive the most recent message on the queue. We can't remove it
+        // yet, since removing a message doesn't return the message key and we
+        // need that to know when we find our marker message.
         rc = QMHRCVPM(
             &msginfo,        // msginfo
             sizeof(msginfo), // msginfolen
@@ -11384,11 +11394,10 @@ static void clear_joblog_messages(int key) {
             "*SAME",         // action
             &err
         );
-
         if (rc != 0) break;
 
+        // Now that we have the message key, remove the message
         outkey = msginfo.key;
-
         rc = QMHRCVPM(
             &msginfo,        // msginfo
             sizeof(msginfo), // msginfolen
@@ -11401,8 +11410,9 @@ static void clear_joblog_messages(int key) {
             "*REMOVE",       // action
             &err
         );
-
         if (rc != 0) break;
+
+        // When we've found our marker message, we're done
         if (outkey == key) break;
     }