[flang] Fix crash in fuzzed input program (#122193)

Fixes #121971.

Author: klausler

flang/lib/Evaluate/shape.cpp

@@ -566,13 +566,13 @@ MaybeExtentExpr GetExtent(const Subscript &subscript, const NamedEntity &base,
                 MaybeExtentExpr{triplet.stride()});
           },
           [&](const IndirectSubscriptIntegerExpr &subs) -> MaybeExtentExpr {
-            if (auto shape{GetShape(subs.value())}) {
-              if (GetRank(*shape) > 0) {
-                CHECK(GetRank(*shape) == 1); // vector-valued subscript
-                return std::move(shape->at(0));
-              }
+            if (auto shape{GetShape(subs.value())};
+                shape && GetRank(*shape) == 1) {
+              // vector-valued subscript
+              return std::move(shape->at(0));
+            } else {
+              return std::nullopt;
             }
-            return std::nullopt;
           },
       },
       subscript.u);

flang/test/Semantics/bug121971.f90

@@ -0,0 +1,10 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+subroutine subr(a,b,n,m)
+  real n,m
+!ERROR: Must have INTEGER type, but is REAL(4)
+!ERROR: Must have INTEGER type, but is REAL(4)
+  integer a(n,m)
+!ERROR: Rank of left-hand side is 2, but right-hand side has rank 1
+!ERROR: Subscript expression has rank 2 greater than 1
+  a = a(a,j)
+end