BUG #18231: String conversion function is adding special characters in Postgres 15

The following bug has been logged on the website:

Bug reference: 18231
Logged by: anvesh work
Email address: work(dot)anvesh(at)gmail(dot)com
PostgreSQL version: 15.0
Operating system: Windows 10
Description:

I have a function which was working fine in Postgres 10 but not in Postgres
15.

When I traced the program, I am facing issue with the VARDATA(arg2).
The arg2 is 2nd argument used in the calling function from PGAdmin4.
After conversion it is returning text with special characters.
In Postgres 10, note that 4 characters are concatenated with special
characters, but other characters are working fine.
Following are the lines of code:
pgsql:
select functionname('arg1', 'arg2', 'arg3');

My Method in C program has
text *arg2 = PG_GETARG_TEXT_P(1);
int output = PbValue (VARDATA(arg2));

Postgres.h file:
#define VARDATA(PTR) VARDATA_4B(PTR)
#define VARDATA_4B(PTR) (((varattrib_4b *) (PTR))->va_4byte.va_data)

To resolve this, we have used memcpy for converting characters.
memcpy(str, VARDATA(arg2), arg_size);