Added return statements after breaking asserts to avoid compiler warn…

…ings. Fixes #996.

vunit/vhdl/data_types/src/api/external_integer_vector_pkg.vhd

@@ -37,11 +37,13 @@ package body external_integer_vector_pkg is
     i  : integer
   ) return integer is begin
     assert false report "EXTERNAL read_integer" severity failure;
+    return integer'low;
   end;
 
   impure function get_ptr (
     id : integer
   ) return extintvec_access_t is begin
     assert false report "EXTERNAL get_intvec_ptr" severity failure;
+    return null;
   end;
 end package body;

vunit/vhdl/data_types/src/api/external_string_pkg.vhd

@@ -37,11 +37,13 @@ package body external_string_pkg is
     i  : integer
   ) return character is begin
     assert false report "EXTERNAL read_char" severity failure;
+    return NUL;
   end;
 
   impure function get_ptr (
     id : integer
   ) return extstring_access_t is begin
     assert false report "EXTERNAL get_string_ptr" severity failure;
+    return null;
   end;
 end package body;

vunit/vhdl/data_types/src/string_ptr_pkg-body-2002p.vhd

@@ -285,6 +285,7 @@ package body string_ptr_pkg is
         when others =>
           -- @TODO Implement to_string for external models
           check_external(ref, "to_string");
+          return "";
       end case;
     end;