Fixed finding variables defined in the upper proc failed

If no new variable was defined in the `eval` method, the variable was hidden from the nested `eval` method.

```ruby
a = 1
p eval %(b = 2; eval %(a)) # => 1 (good)
p eval %(eval %(a)) # => undefined method 'a' (NoMethodError)
```

This issue has occurred since mruby 3.0.0.

mrbgems/mruby-compiler/core/codegen.c

@@ -807,10 +807,12 @@ search_upvar(codegen_scope *s, mrb_sym id, int *idx)
     int i;
 
     const mrb_sym *v = ir->lv;
-    for (i=1; n > 1; n--, v++, i++) {
-      if (*v == id) {
-        *idx = i;
-        return lv - 1;
+    if (v) {
+      for (i=1; n > 1; n--, v++, i++) {
+        if (*v == id) {
+          *idx = i;
+          return lv - 1;
+        }
       }
     }
     if (MRB_PROC_SCOPE_P(u)) break;

mrbgems/mruby-compiler/core/parse.y

@@ -286,9 +286,10 @@ local_var_p(parser_state *p, mrb_sym sym)
     const mrb_sym *v = ir->lv;
     int i;
 
-    if (!v) break;
-    for (i=0; i+1 < ir->nlocals; i++) {
-      if (v[i] == sym) return TRUE;
+    if (v) {
+      for (i=0; i+1 < ir->nlocals; i++) {
+        if (v[i] == sym) return TRUE;
+      }
     }
     if (MRB_PROC_SCOPE_P(u)) break;
     u = u->upper;