[Bug middle-end/85602] -Wsizeof-pointer-memaccess for strncat with size of source

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85602

--- Comment #7 from Martin Sebor <msebor at gcc dot gnu.org> ---
Adjusted patch committed into trunk in r261718.