c: Fix crash in c-typeck.cc convert_arguments with indirect calls

gcc/c/

	PR c/120055
	* c-typeck.cc (convert_arguments): Check if fundecl is null
	before checking for builtin function declaration.

gcc/testsuite/

	* gcc.dg/Wdeprecated-non-prototype-6.c: New test.

(cherry picked from commit 02fa088f5b61fb5ddfff9e2dc0c0404450e7c6a4)