Base+LibWeb: Stub out negative spans

Ensure that when a grid item is passed with a span and a fixed end
position, that if the resulting start of this item is less than 0 then
it won't throw. This is a temporary measure until the correct
functionality is implemented.

Base/res/html/misc/display-grid.html

@@ -40,6 +40,14 @@
   <div class="grid-item">1</div>
 </div>
 
+<!-- Spans causing positions outside the inherit grid. (span 2 with an end position of 1 causes the start to be -1) -->
+<p>If you can see this message then the test passed.</p>
+<div class="grid-container">
+  <div class="grid-item" style="grid-row: span 2 / 1; grid-column: span 2 / 1;">1</div>
+  <div class="grid-item" style="grid-row: span 2 / 1;">2</div>
+  <div class="grid-item" style="grid-column: span 2 / 1;">3</div>
+</div>
+
 <!-- Different column sizes -->
 <p>Should render a 2x2 grid with columns 50px and 50%</p>
 <div 

Userland/Libraries/LibWeb/Layout/GridFormattingContext.cpp

@@ -225,6 +225,9 @@ void GridFormattingContext::run(Box const& box, LayoutMode, AvailableSpace const
         if (child_box.computed_values().grid_row_end().is_position() && child_box.computed_values().grid_row_start().is_span()) {
             row_span = child_box.computed_values().grid_row_start().raw_value();
             row_start = row_end - row_span;
+            // FIXME: Remove me once have implemented spans overflowing into negative indexes, e.g., grid-row: span 2 / 1
+            if (row_start < 0)
+                row_start = 1;
         }
 
         // If a name is given as a <custom-ident>, only lines with that name are counted. If not enough
@@ -354,6 +357,9 @@ void GridFormattingContext::run(Box const& box, LayoutMode, AvailableSpace const
             if (child_box.computed_values().grid_column_end().is_position() && child_box.computed_values().grid_column_start().is_span()) {
                 column_span = child_box.computed_values().grid_column_start().raw_value();
                 column_start = column_end - column_span;
+                // FIXME: Remove me once have implemented spans overflowing into negative indexes, e.g., grid-column: span 2 / 1
+                if (column_start < 0)
+                    column_start = 1;
             }
 
             // If a name is given as a <custom-ident>, only lines with that name are counted. If not enough