patch 8.1.0285: compiler warning for conversion

Problem:    Compiler warning for conversion.
Solution:   Add a type cast. (Mike Williams)
diff --git a/src/ex_getln.c b/src/ex_getln.c
index ba6fe72..d200eab 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -580,7 +580,7 @@
     {
 	pat = last_search_pattern();
 	skiplen = 0;
-	patlen = STRLEN(pat);
+	patlen = (int)STRLEN(pat);
     }
     else
 	pat = ccline.cmdbuff + skiplen;