how to null terminate a string in assembly language?

Question

I am wondering how can I properly null-terminate a string in assembly language: what I did was simply setting movq $0, (position_to_terminate) and I thought this will terminate my string. However, there are still a lot of stuff after the null-terminator when I print the buffer out:

58   ASSERT(0 == strcmp(buf, "48"));
(gdb) print buf
$1 = "48\000\000\000\000\000\000\000\000\246\367\377\177\000"
(gdb) n

Program received signal SIGSEGV, Segmentation fault.
0x0000000000400a78 in testFormatByteAsHex (objs=objs@entry=0x603690)

I am really confused by this because this works in c, is there any other way to terminate a string in assembly language? Thank you in advance.This is the assembly function that I wrote: The purpose of this function is to read 2 parameters, first one being a value from 0 to 255(%rdi) and the second one being an array of length>=2(%rsi) and store the hexadecimal representation in the %rsi array.

hex_format_byte_as_hex:
    subq $8,%rsp
    movq $0,%r10
    movq $0,%rdx
    

.Lhex_format:
    cmpq $1,%r10
    jg .Lreturn1
    
    movq %rdi,%rax /*store input into %rax*/
    movq $16,%rbx  /*store 16 into %rbx*/
    idivq %rbx     /*store dividend in rax, and remainder in rdx*/

    cmpq $10,%rdx
    jge .Lhex
    addq $48,%rdx
    jmp .Lstore

.Lhex:
    addq $87,%rdx
.Lstore:
    movq %rdx,(%rsi,%r10) /*store remainder in %rsi[0]*/
    addq $1,%r10 /*increase %r10 by 1*/
    movq $0,%rdx
    movq %rax,%rdi
    
    jmp .Lhex_format

.Lreturn1:
    
    movq $0,%r11
    movq $1,%r10
    
    movq (%rsi,%r10),%r12
    movq (%rsi,%r11),%r13
    movq %r12,(%rsi,%r11)
    movq %r13,(%rsi,%r10)

    
    
    movq $2,%r10
    movb $0,(%rsi,%r10)
    addq $8,%rsp


    ret