fix #2060 by handling blank search pattern

frontend/apps/reader/modules/readersearch.lua

@@ -75,6 +75,8 @@ function ReaderSearch:onShowSearchDialog(text)
 end
 
 function ReaderSearch:search(pattern, origin)
+    DEBUG("search pattern", pattern)
+    if pattern == nil or pattern == '' then return end
     local direction = self.direction
     local case = self.case_insensitive
     local page = self.view.state.page

frontend/document/koptinterface.lua

@@ -1087,6 +1087,7 @@ local function all_matches(boxes, pattern, caseInsensitive)
     local function match(i, j)
         local pindex = 1
         local matched_indices = {}
+        if #plist == 0 then return end
         while true do
             if #boxes[i] < j then
                 j = j - #boxes[i]