Support non-constant array sizes in zero initializer

We can use ARRAY_OF for those, just like we do for infinitely large
arrays.

Fixes: #4603

Author: tautschnig

regression/cbmc-library/calloc-02/main.c

@@ -6,4 +6,10 @@ int main()
   char *p = calloc(-1, -1);
   if(p)
     assert(p[0] == 0);
+
+  size_t size;
+  size_t num;
+  p = calloc(size, num);
+  if(p && size > 0 && num > 0)
+    assert(p[0] == 0);
 }

src/util/expr_initializer.cpp

@@ -132,7 +132,8 @@ optionalt<exprt> expr_initializert<nondet>::expr_initializer_rec(
       if(!tmpval.has_value())
         return {};
 
-      if(array_type.size().id()==ID_infinity)
+      const auto array_size = numeric_cast<mp_integer>(array_type.size());
+      if(array_type.size().id() == ID_infinity || !array_size.has_value())
       {
         if(nondet)
           return side_effect_expr_nondett(type, source_location);
@@ -142,15 +143,6 @@ optionalt<exprt> expr_initializert<nondet>::expr_initializer_rec(
         return std::move(value);
       }
 
-      const auto array_size = numeric_cast<mp_integer>(array_type.size());
-      if(!array_size.has_value())
-      {
-        if(nondet)
-          return side_effect_expr_nondett(type, source_location);
-        else
-          return {};
-      }
-
       if(*array_size < 0)
         return {};