Make nsFind::Find handle empty string searches.

embedding/components/find/nsFind.cpp

@@ -966,6 +966,11 @@ nsFind::Find(const char16_t* aPatText, nsIDOMRange* aSearchRange,
 
   const char16_t* patStr = patAutoStr.get();
   int32_t patLen = patAutoStr.Length() - 1;
+  
+  // If this function is called with an empty string, we should early exit.
+  if (patLen < 0) {
+    return NS_OK;
+  }
 
   // current offset into the pattern -- reset to beginning/end:
   int32_t pindex = (mFindBackward ? patLen : 0);