fix: make parameter name fallback null-safe in Predicate::formatSummaryCall

[Snezhkko]

Reason: Formatting function calls in counterexamples could dereference a null parameter pointer when a parameter is absent and no concrete value is available, leading to undefined behavior.

Change: Guard against null params.at(i) and use a safe fallback: prefer the parameter’s actual name when available, otherwise use the placeholder param_. The existing “counterexample incomplete” note remains unchanged.

Goal: Eliminate potential UB and make counterexample formatting robust. This aligns with existing null checks in nearby code (e.g., CHC.cpp, RenameSymbol.cpp) and follows existing placeholder naming (param_) used elsewhere. Tests pass.

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[clonker]

It is subtle but the first if statement contains params.at(i). This throws if i is not in the map and exits from the control flow. Therefore subsequent uses of the operator[] on params are safe. For function calls the pointers contained in params can't be null by virtue of how they are constructed in the solidity parser. There is no type-level guarantee for this of course. Did you encounter a null pointer in this anywhere?