patch 8.1.2083: multi-byte chars do not work properly with "%.*S" in printf()

Problem:    Multi-byte chars do not work properly with "%.*S" in printf().
Solution:   Use mb_ptr2cells(). Daniel Hahler, closes #4989)

src/message.c

@@ -4405,12 +4405,16 @@ vim_vsnprintf_typval(
 				     - mb_string2cells((char_u *)str_arg, -1);
 			if (precision)
 			{
-			    char_u *p1 = (char_u *)str_arg;
-			    size_t i;
-
-			    for (i = 0; i < precision && *p1; i++)
-				p1 += mb_ptr2len(p1);
+			    char_u  *p1;
+			    size_t  i = 0;
 
+			    for (p1 = (char_u *)str_arg; *p1;
+							  p1 += mb_ptr2len(p1))
+			    {
+				i += (size_t)mb_ptr2cells(p1);
+				if (i > precision)
+				    break;
+			    }
 			    str_arg_l = precision = p1 - (char_u *)str_arg;
 			}
 		    }

src/testdir/test_expr.vim

@@ -248,6 +248,9 @@ function Test_printf_misc()
   call assert_equal('abc ', printf('%-4s', 'abc'))
   call assert_equal('abc ', printf('%-4S', 'abc'))
 
+  call assert_equal('🐍', printf('%.2S', '🐍🐍'))
+  call assert_equal('', printf('%.1S', '🐍🐍'))
+
   call assert_equal('1%', printf('%d%%', 1))
 endfunc
 

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2083,
 /**/
     2082,
 /**/