[Bug analyzer/108403] New: -Wanalyzer-null-dereference false negative with *q == 0

[mengli.ming at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108403

            Bug ID: 108403
           Summary: -Wanalyzer-null-dereference false negative with *q ==
                    0
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: analyzer
          Assignee: dmalcolm at gcc dot gnu.org
          Reporter: mengli.ming at outlook dot com
  Target Milestone: ---

I got a false negative error when compiling the following program with
gcc(trunk) `-O0 -fanalyzer` in https://godbolt.org/z/4f7e3z8dE.

In this case, after entering the first if branch, the result of eval on line 10
is TRUE. At this time, the fact known to the analyzer is that the pointer `q`
is NULL, and then, for `*q == 0` on line 11, analyzer does not generate an NPD
warning, which is a little odd.

Here is the analysis results of the case. Thank you for taking the time to
review this case.

Input:

```c
#include "stdio.h"
void __analyzer_eval(int);
int *f(int *);

int *f(int *q)
{
    __analyzer_eval(q == 0);
    if (q == 0)
    {
        __analyzer_eval(q == 0);
        if (*q == 0)
        {
            printf("Hello World!");
        }
    }
}

```

Output:

```bash
<source>: In function 'f':
<source>:7:5: warning: UNKNOWN
    7 |     __analyzer_eval(q == 0);
      |     ^~~~~~~~~~~~~~~~~~~~~~~
<source>:10:9: warning: TRUE
   10 |         __analyzer_eval(q == 0);
      |         ^~~~~~~~~~~~~~~~~~~~~~~

```