Merge pull request #16488 from vpisarev:filestorage_longlines

trying to fix handling file storages with extremely long lines

* trying to fix handling of file storages with extremely long lines: https://github.com/opencv/opencv/issues/11061

* * fixed errorneous pointer access in JSON parser.
* it's now crash-test time! temporarily set the initial parser buffer size to just 40 bytes. let's run all the test and check if the buffer is always correctly resized and handled

* fixed pointer use in JSON parser; added the proper test to catch this case

* fixed the test to make it more challenging. generate test json with
*
**
***
etc. shape

modules/core/src/persistence_json.cpp

@@ -411,7 +411,10 @@ public:
         if( *ptr != '"' )
             CV_PARSE_ERROR_CPP( "Key must end with \'\"\'" );
 
-        const char * end = ptr;
+        if( ptr == beg )
+            CV_PARSE_ERROR_CPP( "Key is empty" );
+        value_placeholder = fs->addNode(collection, std::string(beg, (size_t)(ptr - beg)), FileNode::NONE);
+
         ptr++;
         ptr = skipSpaces( ptr );
         if( !ptr || !*ptr )
@@ -420,11 +423,6 @@ public:
         if( *ptr != ':' )
             CV_PARSE_ERROR_CPP( "Missing \':\' between key and value" );
 
-        /* [beg, end) */
-        if( end <= beg )
-            CV_PARSE_ERROR_CPP( "Key is empty" );
-
-        value_placeholder = fs->addNode(collection, std::string(beg, (size_t)(end - beg)), FileNode::NONE);
         return ++ptr;
     }
 
@@ -768,8 +766,6 @@ public:
             CV_PARSE_ERROR_CPP( "left-brace of top level is missing" );
         }
 
-        if( !ptr || !*ptr )
-            CV_PARSE_ERROR_CPP( "Unexpected End-Of-File" );
         return true;
     }