patch 8.2.2730: Coverity complains about not restoring character

Problem:    Coverity complains about not restoring character.
Solution:   Also restore the character in case of an error.
diff --git a/src/vim9compile.c b/src/vim9compile.c
index c9ca595..feb1838 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -6108,6 +6108,7 @@
 	{
 	    // this should not happen
 	    emsg(_(e_missbrac));
+	    var_start[varlen] = c;
 	    return FAIL;
 	}
 	var_start[varlen] = c;