Fix bug related to string_type move fortran-lang#731 fortran-lang#735

Author: jvdp1

Diff for: src/stdlib_string_type.fypp

@@ -680,9 +680,18 @@ contains
     !> No output
     elemental subroutine move_string_string(from, to)
         type(string_type), intent(inout) :: from
-        type(string_type), intent(out) :: to
+        type(string_type), intent(inout) :: to
+
+        if(.not.allocated(from%raw))then
+         if(allocated(to%raw))deallocate(to%raw)
+         return
+        endif
 
-        call move_alloc(from%raw, to%raw)
+        if(from%raw .eq. to%raw)then
+         deallocate(from%raw)
+        else
+         call move_alloc(from%raw, to%raw)
+        endif
 
     end subroutine move_string_string