patch 8.2.4070: using uninitialized memory when reading empty file

Problem:    Using uninitialized memory when reading empty file.
Solution:   Check for empty file before checking for NL. (Dominique Pellé,
            closes #9511)

src/filepath.c

@@ -1796,7 +1796,7 @@ read_file_or_blob(typval_T *argvars, typval_T *rettv, int always_blob)
 		p < buf + readlen || (readlen <= 0 && (prevlen > 0 || binary));
 		++p)
 	{
-	    if (*p == '\n' || readlen <= 0)
+	    if (readlen <= 0 || *p == '\n')
 	    {
 		listitem_T  *li;
 		char_u	    *s	= NULL;

src/testdir/test_eval_stuff.vim

@@ -93,6 +93,13 @@ func Test_readfile_binary()
   call delete('XReadfile_bin')
 endfunc
 
+func Test_readfile_binary_empty()
+  call writefile([], 'Xempty-file')
+  " This used to compare uninitialized memory in Vim <= 8.2.4065
+  call assert_equal([''], readfile('Xempty-file', 'b'))
+  call delete('Xempty-file')
+endfunc
+
 func Test_readfile_bom()
   call writefile(["\ufeffFOO", "FOO\ufeffBAR"], 'XReadfile_bom')
   call assert_equal(['FOO', 'FOOBAR'], readfile('XReadfile_bom'))

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4070,
 /**/
     4069,
 /**/