patch 8.2.4974: ":so" command may read after end of buffer

Problem:    ":so" command may read after end of buffer.
Solution:   Compute length of text properly.
diff --git a/src/version.c b/src/version.c
index 68d11a2..98bb40d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4974,
+/**/
     4973,
 /**/
     4972,