fix(float): fix potential heap corruption in win_redr_border

src/nvim/screen.c

@@ -5655,7 +5655,9 @@ static void win_redr_border(win_T *wp)
       int ic = (i == 0 && !adj[3] && chars[6][0]) ? 6 : 5;
       grid_put_schar(grid, irow+adj[0], i+adj[3], chars[ic], attrs[ic]);
     }
-    grid_put_schar(grid, irow+adj[0], icol+adj[3], chars[4], attrs[4]);
+    if (adj[1]) {
+      grid_put_schar(grid, irow+adj[0], icol+adj[3], chars[4], attrs[4]);
+    }
     grid_puts_line_flush(false);
   }
 }

test/functional/ui/float_spec.lua

@@ -1194,6 +1194,50 @@ describe('float window', function()
         ]]}
       end
 
+      meths.win_set_config(win, {border={"", "_", "", "", "", "-", "", ""}})
+      if multigrid then
+        command('redraw!') -- TODO: without a redraw this test fails, which should be fixed
+        screen:expect{grid=[[
+        ## grid 1
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [2:----------------------------------------]|
+          [3:----------------------------------------]|
+        ## grid 2
+          ^                                        |
+          {0:~                                       }|
+          {0:~                                       }|
+          {0:~                                       }|
+          {0:~                                       }|
+          {0:~                                       }|
+        ## grid 3
+                                                  |
+        ## grid 5
+          {5:_________}|
+          {1: halloj! }|
+          {1: BORDAA  }|
+          {5:---------}|
+        ]], float_pos={
+          [5] = { { id = 1002 }, "NW", 1, 2, 5, true }
+        }, win_viewport={
+          [2] = {win = {id = 1000}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1};
+          [5] = {win = {id = 1002}, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2};
+        }}
+      else
+        screen:expect{grid=[[
+          ^                                        |
+          {0:~                                       }|
+          {0:~    }{5:_________}{0:                          }|
+          {0:~    }{1: halloj! }{0:                          }|
+          {0:~    }{1: BORDAA  }{0:                          }|
+          {0:~    }{5:---------}{0:                          }|
+                                                  |
+        ]]}
+      end
+
       insert [[
         neeed some dummy
         background text