infcall: move assertions in 'call_function_by_hand_dummy' to an earlier spot

This is a refactoring that performs type assertions on the callee
function at the beginning of 'call_function_by_hand_dummy' rather than
at a later point so that

- the checks are grouped together at the beginning of the function for
improved readability, and

- we don't have to align and push things on the stack only to find out
later that the function call is illegal.

gdb/ChangeLog:
2019-10-23  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* infcall.c (call_function_by_hand_dummy): Refactor.

Change-Id: I411ac083ac6a9ee6eb93c4b82393a81a4fc927be