patch 8.2.4840: heredoc expression evaluated even when skipping

Problem:    Heredoc expression evaluated even when skipping.
Solution:   Don't evaluate when "skip" is set. (closes #10306)

src/evalvars.c

@@ -835,7 +835,7 @@ heredoc_get(exarg_T *eap, char_u *cmd, int script_get, int vim9compile)
 	}
 	else
 	{
-	    if (evalstr)
+	    if (evalstr && !eap->skip)
 	    {
 		str = eval_all_expr_in_str(str);
 		if (str == NULL)

src/testdir/test_let.vim

@@ -509,20 +509,24 @@ func Test_let_heredoc_eval()
     let c = "abc`=str`d"
   END
   call assert_equal(['let a = 15', 'let b = 6 + 6', '', 'let c = "abcd"'], code)
+
   let $TESTVAR = "Hello"
   let code =<< eval trim END
     let s = "`=$TESTVAR`"
   END
   call assert_equal(['let s = "Hello"'], code)
+
   let code =<< eval END
     let s = "`=$TESTVAR`"
 END
   call assert_equal(['    let s = "Hello"'], code)
+
   let a = 10
   let data =<< eval END
 `=a`
 END
   call assert_equal(['10'], data)
+
   let x = 'X'
   let code =<< eval trim END
     let a = `abc`
@@ -530,12 +534,14 @@ END
     let c = `
   END
   call assert_equal(['let a = `abc`', 'let b = X', 'let c = `'], code)
+
   let code = 'xxx'
   let code =<< eval trim END
     let n = `=5 +
     6`
   END
   call assert_equal('xxx', code)
+
   let code =<< eval trim END
      let n = `=min([1, 2]` + `=max([3, 4])`
   END
@@ -562,6 +568,13 @@ END
   LINES
   call v9.CheckScriptFailure(lines, 'E15:')
 
+  " skipped heredoc
+  if 0
+    let msg =<< trim eval END
+        n is: `=n`
+    END
+  endif
+
   " Test for sourcing a script containing a heredoc with invalid expression.
   " Variable assignment should fail, if expression evaluation fails
   new

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4840,
 /**/
     4839,
 /**/