Also check for pointers whose width is too small

This ensures the other preconditions don't pass spuriously with a zero
width pointer

Author: Thomas Kiley

src/ansi-c/ansi_c_internal_additions.cpp

@@ -133,6 +133,7 @@ using max_alloc_sizet = uint64_t;
 static max_alloc_sizet
 max_malloc_size(std::size_t pointer_width, std::size_t object_bits)
 {
+  PRECONDITION(pointer_width >= 1);
   PRECONDITION(object_bits < pointer_width - 1);
   PRECONDITION(object_bits >= 1);
   const auto bits_for_offset = pointer_width - object_bits - 1;

unit/ansi-c/max_malloc_size.cpp

@@ -27,6 +27,11 @@ TEST_CASE(
     REQUIRE_THROWS_AS(max_malloc_size(4, 0), invariant_failedt);
   }
 
+  SECTION("Not enough bits in the pointer")
+  {
+    REQUIRE_THROWS_AS(max_malloc_size(0, 0), invariant_failedt);
+  }
+
   SECTION("Max allocation size overflow")
   {
     REQUIRE_THROWS_AS(max_malloc_size(128, 63), invariant_failedt);