strbuf: several cleanups for strbuf_add_string()

strbuf: several cleanups for strbuf_add_string()

- add missing assertions,
- use GREEDY_REALLOC() at one more place,
- etc.

Before:
```
$ sudo time valgrind --leak-check=full ./systemd-hwdb update
==112572== Memcheck, a memory error detector
==112572== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==112572== Using Valgrind-3.23.0 and LibVEX; rerun with -h for copyright info
==112572== Command: ./systemd-hwdb update
==112572==
==112572==
==112572== HEAP SUMMARY:
==112572==     in use at exit: 0 bytes in 0 blocks
==112572==   total heap usage: 1,320,113 allocs, 1,320,113 frees, 70,614,501 bytes allocated
==112572==
==112572== All heap blocks were freed -- no leaks are possible
==112572==
==112572== For lists of detected and suppressed errors, rerun with: -s
==112572== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
21.94user 0.19system 0:22.23elapsed 99%CPU (0avgtext+0avgdata 229876maxresident)k
0inputs+25264outputs (0major+57275minor)pagefaults 0swaps
```

After:
```
$ sudo time valgrind --leak-check=full ./systemd-hwdb update
[sudo] password for watanabe:
==114732== Memcheck, a memory error detector
==114732== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==114732== Using Valgrind-3.23.0 and LibVEX; rerun with -h for copyright info
==114732== Command: ./systemd-hwdb update
==114732==
==114732==
==114732== HEAP SUMMARY:
==114732==     in use at exit: 0 bytes in 0 blocks
==114732==   total heap usage: 1,276,406 allocs, 1,276,406 frees, 68,500,491 bytes allocated
==114732==
==114732== All heap blocks were freed -- no leaks are possible
==114732==
==114732== For lists of detected and suppressed errors, rerun with: -s
==114732== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
21.91user 0.24system 0:22.26elapsed 99%CPU (0avgtext+0avgdata 233584maxresident)k
0inputs+25168outputs (0major+58237minor)pagefaults 0swaps
```