fix(box): calculate gc memory pool size after calling a function

A function, evaluated by a module_func_call(), can have side effects on the fiber's GC memory pool. In an original case some SQL statements were prepared and later marked for GC. As a result if we calculate the size of GC memory pool before calling a function and later used this value to free invalid memory size and failed on an assert.

So, lets estimate the fiber's GC memory pool right after calling an arbitrary function.

NO_DOC=refactoring NO_TEST=refactoring NO_CHANGELOG=refactoring