fix: add null guards in IFDS solver to prevent NPE crashes on large APKs

[JLEnoch1]

…prevent NPE crashes on large APKs

Two NullPointerExceptions in the IFDS solver crash FlowDroid on large/complex APKs (e.g., 46 MB, 336K+ methods, 6 DEX):

1. targetVal=null NPE in IFDSSolver.propagate(): When the alias solver callback injects an edge with a null abstraction, targetVal.getPathLength() throws NPE and kills the entire analysis. The memoryManager block only checks for null inside its own scope, so when memoryManager is null, targetVal is never validated.

2. targets=null NPE in FlowInsensitiveSolver.processExit(): computeReturnFlowFunction() can return null (the FlowFunction contract allows it), but the main branch of processExit() iterates over the result without a null check. The followReturnsPastSeeds branch already has @this guard.

Fix: Add early-return null guards before the dereference points in all three solver variants (fastSolver, gcSolver, flowInsensitive).
Closes #879

[StevenArzt]

This merge request solves the problem, and I think it's good to have these checks for additional stability. At the same time, the checks hide the underlying problem. The alias solver should not inject null abstractions. The flow functions should not add null elements to the list of abstractions either.

If you have some time, please look into the root cause of this issue.