libpq: Remove redundant null pointer checks before free()

libpq contains a lot of

if (foo)
free(foo);

calls, where the "if" part is unnecessary. This is of course pretty
harmless, but some functions like scram_free() and freePGconn() have
become so bulky that it becomes annoying. So while I was doing some
work in that area I undertook to simplify this.