patch 8.0.0512: check for available characters takes too long

Problem:    Check for available characters takes too long.
Solution:   Only check did_start_blocking if wtime is negative. (Daisuke
            Suzuki, closes #1591)
diff --git a/src/version.c b/src/version.c
index 7d2cf27..3ebe8a6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    512,
+/**/
     511,
 /**/
     510,