PHALCON_INIT_NVAR(_PNULL) optimization

[ghost]

Replaces calls to

zval_ptr_dtor(&z);
PHALCON_ALLOC_ZVAL(z);

with

zval_dtor(z);
ZVAL_NULL(z);

when Z_REFCOUNT_P(z) == 1.

This:

• makes sure that the new variable will not changes its location in memory (and thus PHALCON_INIT_NVAR() can be used after PHALCON_SEPARATE_PARAM() — this will make Image code more readable — @dreamsxin, please watch this thread!)
• reduces the number of memory (re)allocations

When manual-unit.php is run under valgrind:

• before: 1,994,062 allocs, 1,993,839 frees, 355,849,237 bytes allocated
• after: 1,968,935 allocs, 1,968,712 frees, 355,062,535 bytes allocated

[dreamsxin]

OK, I will merge this.

[phalcon]

great!