[flang][cuda] Avoid crash by exiting the check if assignment is not usable (#89149)

In the presence of other semantic error, `GetAssignment` would return a
nullptr and therefore would make the rest of the check crash when trying
to collect symbols.

Exiting early when we have a nullptr so the compiler doesn't crash and
user can get the meaningful semantic error.

Author: clementval

flang/lib/Semantics/check-cuda.cpp

@@ -488,6 +488,10 @@ void CUDAChecker::Enter(const parser::AssignmentStmt &x) {
   }
 
   const evaluate::Assignment *assign{semantics::GetAssignment(x)};
+  if (!assign) {
+    return;
+  }
+
   int nbLhs{evaluate::GetNbOfCUDASymbols(assign->lhs)};
   int nbRhs{evaluate::GetNbOfCUDASymbols(assign->rhs)};
 

flang/test/Semantics/cuf11.cuf

@@ -22,3 +22,11 @@ subroutine sub1()
   ahost = adev + adev
 
 end subroutine
+
+logical function compare_h(a,b)
+!ERROR: Derived type 'h' not found
+  type(h) :: a, b
+!ERROR: 'a' is not an object of derived type; it is implicitly typed
+!ERROR: 'b' is not an object of derived type; it is implicitly typed
+  compare_h = (a%h .eq. b%h)
+end