patch 8.2.0018: :join does not add white space where it should

Problem:    :join does not add white space where it should. (Zdenek Dohnal)
Solution:   Handle joining multiple lines propely.

src/ops.c

@@ -2001,7 +2001,7 @@ do_join(
 	{
 	    curr = skipwhite(curr);
 	    if (*curr != NUL && *curr != ')'
-		    && currsize != 0 && endcurr1 != TAB
+		    && sumsize != 0 && endcurr1 != TAB
 		    && (!has_format_option(FO_MBYTE_JOIN)
 			|| (mb_ptr2char(curr) < 0x100 && endcurr1 < 0x100))
 		    && (!has_format_option(FO_MBYTE_JOIN2)

src/testdir/test_join.vim

@@ -427,3 +427,14 @@ int i = 7 /* foo *// 3
   call assert_equal(expected, getline(1, '$'))
   close!
 endfunc
+
+func Test_join_lines()
+  new
+  call setline(1, ['a', 'b', '', 'c', 'd'])
+  %join
+  call assert_equal('a b c d', getline(1))
+  call setline(1, ['a', 'b', '', 'c', 'd'])
+  normal 5J
+  call assert_equal('a b c d', getline(1))
+  bwipe!
+endfunc

src/version.c

@@ -742,6 +742,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    18,
 /**/
     17,
 /**/