Use vector::at() rather than operator[]

[kevinbackhouse]

Fixes #1706

The problem in #1706 is that PentaxMakerNote::printDate() expects a value of type undefined, but it is given a value of type unsignedLong instead. That leads to an out-of-bounds access of a std::vector, which triggers an assertion failure.

It looks difficult to prevent printDate, and similar functions like printTime, from getting called on values with the wrong type. So I think the most reliable way of preventing a crash is by using vector::at() rather than operator[]. The difference is that vector::at() throws an exception if the index is out-of-bounds. It is easy to catch the exception and handle the error gracefully.

[hassec]

LGTM! 👍

[kevinbackhouse]

This is the CodeQL query that I used to find the similar value_[n] accesses to the one that caused the bug:

import cpp
import semmle.code.cpp.rangeanalysis.SimpleRangeAnalysis

from FunctionCall call, ClassTemplateInstantiation t, TemplateClass c, string path
where
  call.getQualifier().(VariableAccess).getTarget().getName() = "value_" and
  call.getTarget().getName() = "operator[]" and
  t = call.getQualifier().getType().getUnspecifiedType() and
  c = t.getTemplate() and
  c.getSimpleName() != "map" and
  not exists (Expr arg |
    c.getSimpleName() = "array" and arg = call.getArgument(0)
    and lowerBound(arg) >= 0 and
    upperBound(arg) < t.getTemplateArgument(1).(Literal).getValue().toInt()) and
  path = call.getLocation().getFile().getRelativePath() and
  not path.matches("xmpsdk/%")
select call, t, c, path