[flang] Fix character length checking in ALLOCATE (#163657)

The known character length compatibility check for ALLOCATE statements
needs to allow for negative lengths, which are effectively zero.

Fixes llvm/llvm-project#163242.

Author: klausler

flang/lib/Semantics/check-allocate.cpp

@@ -441,7 +441,7 @@ static bool HaveCompatibleLengths(
         evaluate::ToInt64(type1.characterTypeSpec().length().GetExplicit())};
     auto v2{
         evaluate::ToInt64(type2.characterTypeSpec().length().GetExplicit())};
-    return !v1 || !v2 || *v1 == *v2;
+    return !v1 || !v2 || (*v1 >= 0 ? *v1 : 0) == (*v2 >= 0 ? *v2 : 0);
   } else {
     return true;
   }
@@ -454,7 +454,7 @@ static bool HaveCompatibleLengths(
     auto v1{
         evaluate::ToInt64(type1.characterTypeSpec().length().GetExplicit())};
     auto v2{type2.knownLength()};
-    return !v1 || !v2 || *v1 == *v2;
+    return !v1 || !v2 || (*v1 >= 0 ? *v1 : 0) == (*v2 >= 0 ? *v2 : 0);
   } else {
     return true;
   }

flang/test/Semantics/bug163242.f90

@@ -0,0 +1,5 @@
+!RUN: %flang -fc1 -fsyntax-only %s | FileCheck --allow-empty %s
+!CHECK-NOT: error:
+character(0), allocatable :: ch
+allocate(character(-1) :: ch)
+end