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/fileio.c b/src/fileio.c
index 700bd3e..35f4fd2 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -1392,7 +1392,7 @@
 
 			/* If the crypt layer is buffering, not producing
 			 * anything yet, need to read more. */
-			if (size > 0 && decrypted_size == 0)
+			if (decrypted_size == 0)
 			    continue;
 
 			if (linerest == 0)