NULL values in field records are not supported

[fdie]

Issue analysis

p1_mysql_bin.erl:

decode_resultset_row(LogFun, [Type | TypeRest], Column, Null, Value, Acc) ->
    case (1 bsl Column) band Null of
   	    1 -> ...   	    

• Obviously, this code is wrong since it could be 1 for Column 0 only, but any column could have a "NULL" value...

• We had the below crash because a record of a table had a last column with a "NULL" value,
  resulting in a correct decoding of all columns but the last column since
  a string value decoding is attempted but fails (due to lack of data, see <<>>).
  This column is erroneously not considered having a "NULL" value due to a "NULL bitmap" handling issue.

** {function_clause,                                                                                                                        
       [{p1_mysql_bin,decode_var_int,                                 
            [<<>>],                                                   
            [{file,"/build/deps/p1_mysql/src/p1_mysql_bin.erl"},{line,194}]},                                                               
        {p1_mysql_bin,decode_var_string,1,                            
            [{file,"/build/deps/p1_mysql/src/p1_mysql_bin.erl"},{line,181}]},                                                               
        {p1_mysql_bin,decode_resultset_row,6,                         
            [{file,"/build/deps/p1_mysql/src/p1_mysql_bin.erl"},{line,308}]},                                                               
        {p1_mysql_bin,get_resultset_rows,4,                           
            [{file,"/build/deps/p1_mysql/src/p1_mysql_bin.erl"},{line,318}]},                                                               
        {p1_mysql_bin,get_execute_stmt_response,3,                    
            [{file,"/build/deps/p1_mysql/src/p1_mysql_bin.erl"},{line,136}]},                                                               
        {p1_mysql_bin,execute,5,                                      
            [{file,"/build/deps/p1_mysql/src/p1_mysql_bin.erl"},{line,78}]},                                                                
        {p1_mysql_conn,handle_info,2,                                 
            [{file,"src/p1_mysql_conn.erl"},{line,408}]},             
        {gen_server,try_dispatch,4,[{file,"gen_server.erl"},{line,1123}]}]} 

The issue is due to a wrong decoding of the "NULL" bitmap.
The spec https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_binary_resultset.html
describes a "NULL bitmap" with a bit for each column index and with 2 leading bits that should be ignored.
Those 2 bits are not taken into account in the current source code which causes a 2 bits shift
in the bitmap integer value.

Patch

We tried a table with records having "NULL" values at position 0, 3 and the last column 5 and the resultset
is correctly decoded once the provided patch has been applied.
Some unit tests of the binary protocol would be helpful ;-)

[prefiks]

Looks good, i will merge it.