Silence -Wmaybe-uninitialized warnings

Hi

When building postgresql with CFLAGS="-Og" gcc emits
'-Wmaybe-uninitialized' warnings at few places. The variables are
initialized on all execution paths but the compiler was not able to
prove it with the additional optimization flag enabled. The patch
initializes the variables with NULL to silence the warnings. No
functional change intended.

```
[646/2085] Compiling C object
src/backend/postgres_lib.a.p/commands_copyfromparse.c.o
../src/backend/commands/copyfromparse.c: In function ‘CopyFromTextOneRow’:
../src/backend/commands/copyfromparse.c:995:39: warning:
‘field_strings’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
995 | string = field_strings[fieldno++];
| ^
../src/backend/commands/copyfromparse.c:961:21: note: ‘field_strings’
was declared here
961 | char **field_strings;
| ^~~~~~~~~~~~~
../src/backend/commands/copyfromparse.c: In function ‘CopyFromCSVOneRow’:
../src/backend/commands/copyfromparse.c:995:39: warning:
‘field_strings’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
995 | string = field_strings[fieldno++];
| ^
../src/backend/commands/copyfromparse.c:961:21: note: ‘field_strings’
was declared here
961 | char **field_strings;
| ^~~~~~~~~~~~~
[889/2085] Compiling C object
src/backend/postgres_lib.a.p/partitioning_partbounds.c.o
../src/backend/partitioning/partbounds.c: In function
‘check_partition_bounds_for_split_range’:
../src/backend/partitioning/partbounds.c:5452:57: warning: ‘datum’ may
be used uninitialized in this function [-Wmaybe-uninitialized]
5452 |
parser_errposition(pstate, exprLocation((Node *) datum)));
|
```
gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0

Regards
Imran Zaheer