[MLIR][Affine] Fix crash with invalid unroll factor (Issue llvm#64321)

Updated LoopUnroll pass to check if unrollFactor passed by the user is invalid, i.e., less than or equal to
zero. If so, emit appropriate message and signal pass failure.
Reporting Issue is llvm#64321

Author: SwapnilGhanshyala

mlir/lib/Dialect/Affine/Transforms/LoopUnroll.cpp

@@ -94,7 +94,12 @@ void LoopUnroll::runOnOperation() {
   func::FuncOp func = getOperation();
   if (func.isExternal())
     return;
-
+  if (unrollFactor <= 0) {
+    emitError(func->getLoc(),
+              "'-affine-loop-unroll=unroll-factor' cannot be less than or "
+              "equal to zero; pass aborted!");
+    return signalPassFailure();
+  }
   if (unrollFull && unrollFullThreshold.hasValue()) {
     // Store short loops as we walk.
     SmallVector<AffineForOp, 4> loops;