Fix last unitialized memory warning

This patch is really not necessary from a functional point of view. It
is only necessary if we want to silence a compiler warning.

Tested on `gcc (GCC) 13.1.1 20230511 (Red Hat 13.1.1-2)`.

After silencing this warning, all I am left with (given my build
configuration) is:

[1667/2280] Compiling C object src/pl/plpgsql/src/plpgsql.so.p/pl_exec.c.o
In file included from ../src/include/access/htup_details.h:22,
                 from ../src/pl/plpgsql/src/pl_exec.c:21:
In function ‘assign_simple_var’,
    inlined from ‘exec_set_found’ at ../src/pl/plpgsql/src/pl_exec.c:8349:2:
../src/include/varatt.h:230:36: warning: array subscript 0 is outside array bounds of ‘char[0]’ [-Warray-bounds=]
  230 |         (((varattrib_1b_e *) (PTR))->va_tag)
      |                                    ^
../src/include/varatt.h:94:12: note: in definition of macro ‘VARTAG_IS_EXPANDED’
   94 |         (((tag) & ~1) == VARTAG_EXPANDED_RO)
      |            ^~~
../src/include/varatt.h:284:57: note: in expansion of macro ‘VARTAG_1B_E’
  284 | #define VARTAG_EXTERNAL(PTR)                            VARTAG_1B_E(PTR)
      |                                                         ^~~~~~~~~~~
../src/include/varatt.h:301:57: note: in expansion of macro ‘VARTAG_EXTERNAL’
  301 |         (VARATT_IS_EXTERNAL(PTR) && !VARTAG_IS_EXPANDED(VARTAG_EXTERNAL(PTR)))
      |                                                         ^~~~~~~~~~~~~~~
../src/pl/plpgsql/src/pl_exec.c:8537:17: note: in expansion of macro ‘VARATT_IS_EXTERNAL_NON_EXPANDED’
 8537 |                 VARATT_IS_EXTERNAL_NON_EXPANDED(DatumGetPointer(newvalue)))
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

From my perspective, this warning definitely seems like a false
positive, but I don't know the code well-enough to say that for certain.

-- 
Tristan Partin
Neon (https://neon.tech)