patch 8.0.1753: various warnings from a static analyser

Problem:    Various warnings from a static analyser
Solution:   Add type casts, remove unneeded conditions. (Christian Brabandt,
            closes #2770)
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index b11bff4..3e369a8 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -1420,7 +1420,7 @@
 	    if (balloonEval != NULL)
 		general_beval_cb(balloonEval, 0);
 	}
-	else if (this_due > 0 && (next_due == -1 || next_due > this_due))
+	else if (next_due == -1 || next_due > this_due)
 	    next_due = this_due;
     }
 #endif