patch 9.0.0056: wrong line number reported when :cexpr fails in :def function

Problem:    Wrong line number reported when :cexpr fails in :def function.
Solution:   Set line_number before executing :cexpr. (closes #10735)

src/testdir/test_vim9_func.vim

@@ -4205,6 +4205,18 @@ def Test_multiple_funcref()
   v9.CheckScriptSuccess(lines)
 enddef
 
+def Test_cexpr_errmsg_line_number()
+  var lines =<< trim END
+      vim9script
+      def Func()
+        var qfl = {}
+        cexpr qfl
+      enddef
+      Func()
+  END
+  v9.CheckScriptFailure(lines, 'E777', 2)
+enddef
+
 " The following messes up syntax highlight, keep near the end.
 if has('python3')
   def Test_python3_command()

src/version.c

@@ -735,6 +735,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    56,
 /**/
     55,
 /**/

src/vim9execute.c

@@ -2786,6 +2786,7 @@ exec_instructions(ectx_T *ectx)
 		    ea.cmdlinep = &iptr->isn_arg.cexpr.cexpr_ref->cer_cmdline;
 		    --ectx->ec_stack.ga_len;
 		    tv = STACK_TV_BOT(0);
+		    SOURCING_LNUM = iptr->isn_lnum;
 		    res = cexpr_core(&ea, tv);
 		    clear_tv(tv);
 		    if (res == FAIL)