patch 9.0.0966: some compilers don't allow a declaration after a label

Problem:    Some compilers don't allow a declaration after a label.
Solution:   Move the declaration to the start of the block. (John Marriott)
diff --git a/src/screen.c b/src/screen.c
index 7c3a6d7..3bf15d6 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -2383,6 +2383,7 @@
     static int	    entered = FALSE;		// avoid recursiveness
     static int	    done_outofmem_msg = FALSE;	// did outofmem message
     int		    retry_count = 0;
+    int		    found_null;
 
 retry:
     /*
@@ -2509,8 +2510,7 @@
 #endif
 
 give_up:
-
-    int found_null = FALSE;
+    found_null = FALSE;
     for (int i = 0; i < p_mco; ++i)
 	if (new_ScreenLinesC[i] == NULL)
 	{