Fix use after free bug in db2_execute
Fix use after free bug in db2_execute

db2_execute calls _php_db2_execute_helper, which allocs curr->value
if it is currently NULL. Later, _php_db2_set_symbol is called
which frees tmp_curr->value, but it does not set it to NULL, which
means on the next time through, _php_db2_execute_helper will skip
allocating new memory and instead re-use a previously freed address.

Found using Valgrind.

php/pecl-database-ibm_db2,php/pecl-database-ibm_db2