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/evalfunc.c b/src/evalfunc.c
index 62dc40e..dd4462d 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -10194,7 +10194,7 @@
 	}
 
 	rettv->vval.v_number = 1;	/* FAIL */
-	if (line == NULL || lnum < 1 || lnum > curbuf->b_ml.ml_line_count + 1)
+	if (line == NULL || lnum > curbuf->b_ml.ml_line_count + 1)
 	    break;
 
 	/* When coming here from Insert mode, sync undo, so that this can be