Add check for when compile returnes null for invalid patterns

Closes #102

Clover/app/src/main/java/org/floens/chan/core/manager/FilterEngine.java

@@ -176,9 +176,11 @@ public class FilterEngine {
 
             if (matcher == null) {
                 Pattern compiledPattern = compile(filter.pattern);
+                if (compiledPattern != null) {
                     matcher = filter.compiledMatcher = compiledPattern.matcher("");
                     Logger.d(TAG, "Resulting pattern: " + filter.compiledMatcher);
                 }
+            }
 
             if (matcher != null) {
                 matcher.reset(text);