Stdlib: Fix Repeat() implementation

[Shaikh-Ubaid]

Bug:

% cat examples/expr5.f90     
program expr2
    implicit none

    character(len=2) :: x1
    character(len=4) :: x
    x1 = "wh"
    x = "what"

    print *, repeat(x1, 3)
    print *, "|"//repeat(x, 3)//"|"

end program
% gfortran examples/expr5.f90
% ./a.out 
 whwhwh
 |whatwhatwhat|
% lfortran_in_main examples/expr5.f90 
www   
|www         |

[Shaikh-Ubaid]

This needs to be rebased on top of #3636.