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.