when calling a SP from C API"

The bug was caused by lack of checks for misuse in mysql_real_query. 
A stored procedure always returns at least one result, which is the 
status of execution of the procedure itself.
This result, or so-called OK packet, is similar to a result
returned by INSERT/UPDATE/CREATE operations: it contains the overall
status of execution, the number of affected rows and the number of
warnings. The client test program attached to the bug did not read this 
result and ivnoked the next query. In turn, libmysql had no check for 
such scenario and mysql_real_query was simply trying to send that query 
without reading the pending response, thus messing up the communication
protocol.

The fix is to return an error from mysql_real_query when it's called
prior to retrieval of all pending results.