C: False positive about "Potential double free" with negative array indices

**Description of the false positive**

CodeQL warns about potential double free, in situations where there's clearly no such situation. It seems to be related to the use of negative indices

**Code samples or links to source code**

Cf https://github.com/MapServer/MapServer/security/code-scanning/3

The code at https://github.com/MapServer/MapServer/blob/0cb56232d4ca0e64d747efa1db602ff08e0ea42f/src/mapparser.c#L1787 (which is C code generated from a Bison grammar)
```
free((yyvsp[-2].strval));
free((yyvsp[0].strval));
```

generates "Memory pointed to by may already have been freed by. ", but this is obviously wrong as the memory locations are disjoint.

Extract of "Show path" in the report:
```
Step 1 pointer to free output argument
Source
src/mapparser.c:1787
}
}

free((yyvsp[-2].strval));
free((yyvsp[0].strval));
}
#line 1791 "/vagrant/mapparser.c" /* yacc.c:1646 */
Step 2 *access to array [post update] [YYSTYPE]
src/mapparser.c:1787
}
}

free((yyvsp[-2].strval));
free((yyvsp[0].strval));
}
#line 1791 "/vagrant/mapparser.c" /* yacc.c:1646 */
Step 3 *access to array [YYSTYPE]
src/mapparser.c:1788
}

free((yyvsp[-2].strval));
free((yyvsp[0].strval));
}
#line 1791 "/vagrant/mapparser.c" /* yacc.c:1646 */
break;
Step 4 strval
Sink
src/mapparser.c:1788
}

free((yyvsp[-2].strval));
free((yyvsp[0].strval));
Memory pointed to by
may already have been freed by
.
}
#line 1791 "/vagrant/mapparser.c" /* yacc.c:1646 */
break;
```