MDL-45535 CSS chunking breaks media rules if CSS invalid (IE)

Invalid CSS with extra } symbols, which previously (by fluke)
worked, was broken by recent chunking improvements. This change
makes the chunking code robust against this (stupid) situation.

lib/csslib.php

@@ -195,6 +195,12 @@ function css_chunk_by_selector_count($css, $importurl, $maxselectors = 4095, $bu
                 }
             } else {
                 $inrule--;
+                // Handle stupid broken CSS where there are too many } brackets,
+                // as this can cause it to break (with chunking) where it would
+                // coincidentally have worked otherwise.
+                if ($inrule < 0) {
+                    $inrule = 0;
+                }
             }
 
             // We are not in a media query, and there is no pending rule, it is safe to split here.