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/os_unix.c b/src/os_unix.c
index 5646560..c89131f 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -441,7 +441,7 @@
 		    /* no character available within "wtime" */
 		    return 0;
 
-		if (wtime < 0)
+		else
 		{
 		    /* no character available within 'updatetime' */
 		    did_start_blocking = TRUE;