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)

src/testdir/test_vimscript.vim

@@ -1642,6 +1642,23 @@ func Test_script_local_func()
   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

src/userfunc.c

@@ -2572,9 +2572,13 @@ ex_function(exarg_T *eap)
 	    }
 	    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;

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    78,
 /**/
     77,
 /**/