patch 8.2.4870: Vim9: expression in :substitute is not compiled

Problem:    Vim9: expression in :substitute is not compiled.
Solution:   Use an INSTR instruction if possible. (closes #10334)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 4646968..eb6dc28 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -5010,6 +5010,10 @@
     int		save_iidx = ectx->ec_iidx;
     int		res;
 
+    // Initialize rettv so that it is safe for caller to invoke clear_tv(rettv)
+    // even when the compilation fails.
+    rettv->v_type = VAR_UNKNOWN;
+
     ectx->ec_instr = tv->vval.v_instr->instr_instr;
     res = exec_instructions(ectx);
     if (res == OK)