patch 8.2.0078: expanding <sfile> works differently the second time

Problem:    Expanding <sfile> works differently the second time.
Solution:   Keep the expanded name when redefining a function. (closes #5425)
diff --git a/src/testdir/test_vimscript.vim b/src/testdir/test_vimscript.vim
index 89112de..c890fb7 100644
--- a/src/testdir/test_vimscript.vim
+++ b/src/testdir/test_vimscript.vim
@@ -1642,6 +1642,23 @@
   enew! | close
 endfunc
 
+func Test_script_expand_sfile()
+  let lines =<< trim END
+    func s:snr()
+      return expand('<sfile>')
+    endfunc
+    let g:result = s:snr()
+  END
+  call writefile(lines, 'Xexpand')
+  source Xexpand
+  call assert_match('<SNR>\d\+_snr', g:result)
+  source Xexpand
+  call assert_match('<SNR>\d\+_snr', g:result)
+
+  call delete('Xexpand')
+  unlet g:result
+endfunc
+
 func Test_compound_assignment_operators()
     " Test for number
     let x = 1
diff --git a/src/userfunc.c b/src/userfunc.c
index a8bc34a..95862fe 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -2572,9 +2572,13 @@
 	    }
 	    else
 	    {
-		// redefine existing function
+		char_u *exp_name = fp->uf_name_exp;
+
+		// redefine existing function, keep the expanded name
 		VIM_CLEAR(name);
+		fp->uf_name_exp = NULL;
 		func_clear_items(fp);
+		fp->uf_name_exp = exp_name;
 #ifdef FEAT_PROFILE
 		fp->uf_profiling = FALSE;
 		fp->uf_prof_initialized = FALSE;
diff --git a/src/version.c b/src/version.c
index 118dad5..4516eb7 100644
--- a/src/version.c
+++ b/src/version.c
@@ -743,6 +743,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    78,
+/**/
     77,
 /**/
     76,