Declare a couple of variables inside not outside a PG_TRY block.

Declare a couple of variables inside not outside a PG_TRY block.

I went through the buildfarm's reports of "warning: variable 'foo'
might be clobbered by 'longjmp' or 'vfork' [-Wclobbered]".  As usual,
none of them are live problems according to my understanding of the
effects of setjmp/longjmp, to wit that local variables might revert
to their values as of PG_TRY entry, due to being kept in registers.
But I did happen to notice that XmlTableGetValue's "cstr" variable
doesn't need to be declared outside the PG_TRY block at all (thus
giving further proof that the -Wclobbered warning has little
connection to real problems).  We might as well move it inside,
and "cur" too, in hopes of eliminating one of the bogus warnings.