patch 8.0.0544: cppcheck warnings

Problem:    Cppcheck warnings.
Solution:   Use temp variable. Change NUL to NULL. Swap conditions. (Dominique
            Pelle)
diff --git a/src/edit.c b/src/edit.c
index ad8b89a..0153f3b 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -9524,7 +9524,7 @@
 #endif
 	    buf[idx++] = c;
 	buf[idx] = NUL;
-	if (end != NUL && STRNCMP(buf, end, idx) == 0)
+	if (end != NULL && STRNCMP(buf, end, idx) == 0)
 	{
 	    if (end[idx] == NUL)
 		break; /* Found the end of paste code. */