[Analyzer] False positive clang-analyzer-core.UndefinedBinaryOperatorResult

Original issue reported in https://github.com/benhoyt/inih/issues/208 and [openconnect/ocserv!437](https://gitlab.com/openconnect/ocserv/-/merge_requests/437).

This looks like a false positive:
```console
$ git clone https://github.com/benhoyt/inih.git
$
$ scan-build clang -c ini.c
scan-build: Using '/usr/lib/llvm-18/bin/clang' for static analysis
ini.c:164:56: warning: The left operand of '!=' is a garbage value [core.UndefinedBinaryOperatorResult]
164 | if (offset == max_line - 1 && line[offset - 1] != '\n') {
| ~~~~~~~~~~~~~~~~ ^
1 warning generated.
scan-build: Analysis run complete.
scan-build: 1 bug found.
scan-build: Run 'scan-view /tmp/scan-build-2026-01-06-201438-11205-1' to examine bug reports.
$
```

1. I don't understand why the analyser would complain about `line` containing garbage in line [164](https://github.com/benhoyt/inih/blob/8e06f6b77b5d4471bdc6d85ada81b67d37354a5c/ini.c#L164):
```c
if (offset == max_line - 1 && line[offset - 1] != '\n') {
```
but not in previous line [141](https://github.com/benhoyt/inih/blob/8e06f6b77b5d4471bdc6d85ada81b67d37354a5c/ini.c#L141):
```c
offset = strlen(line);
```
2. The `line` char array is filled in [`ini_reader_string`](https://github.com/benhoyt/inih/blob/8e06f6b77b5d4471bdc6d85ada81b67d37354a5c/ini.c#L287-L312) and I fail to see why it wouldn't fill `line`/`str` properly through the `strp` pointer:
```c
/* An ini_reader function to read the next line from a string buffer. This
is the fgets() equivalent used by ini_parse_string(). */
static char* ini_reader_string(char* str, int num, void* stream) {
ini_parse_string_ctx* ctx = (ini_parse_string_ctx*)stream;
const char* ctx_ptr = ctx->ptr;
size_t ctx_num_left = ctx->num_left;
char* strp = str;
char c;

if (ctx_num_left == 0 || num < 2)
return NULL;

while (num > 1 && ctx_num_left != 0) {
c = *ctx_ptr++;
ctx_num_left--;
*strp++ = c;
if (c == '\n')
break;
num--;
}

*strp = '\0';
ctx->ptr = ctx_ptr;
ctx->num_left = ctx_num_left;
return str;
}
```

I am running Clang 1.18 as bundled with Ubuntu 24.04:
```console
$ dpkg -l clang-tools
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-=================-==============-============-=================================
ii clang-tools:amd64 1:18.0-59~exp2 amd64 clang-based tools
$
```

Reproduce the warning with `scan-build clang -c ini.c`, then inspect ini.c around line 164 and compare it with the strlen call at line 141. Trace the buffer initialization through ini_reader_string at lines 287-312 and determine why the analyzer reports a garbage value. Done means the reported false positive is explained and the analyzer no longer emits it for this case.

Written by the indexing model from the issue text.