X86_64 Assembly code segfaults and gives stack smashing error - linux

Question

So, for this assignment I have to write an Assembly "function" to be called by C code. The purpose of the function is, given an integer and a memory address (the address of a char array, to be used as a string), convert the integer to a string, which starting address is the memory address that is given.
I'm on Ubuntu Linux, btw.
Here's the Assembly code (I tried to make it using the Linux x86_64 ABI calling conventions)(It is in AT&T syntax):
.global dec
.type dec, #function
.text
dec:
######################### Subroutine prologue
push %rbp # Save the base pointer
movq %rsp, %rbp # Make the stack pointer the new base pointer
push %rdi # Stack parameter 1
push %rsi # Stack parameter 2
push %rbx # Save callee-saved registers
push %r12
push %r13
push %r14
push %r15
######################### Subroutine body
movq %rdi, %rax
xor %rcx, %rcx
addDigit:
cmp $0, %rax
je putMem
xor %rdx, %rdx
mov $10, %ebx
div %ebx
addq $'0', %rdx
pushq %rdx
inc %rcx
jmp addDigit
putMem:
cmp $0, %rcx
je endProg
popq (%rsi)
add $1, %rsi
dec %rcx
jmp putMem
endProg:
movq $0x0, (%rsi)
movq -16(%rbp), %rsi
mov $1, %rax
######################### Subroutine epilogue
popq %r15 # Restore callee-saved registers
popq %r14
popq %r13
popq %r12
popq %rbx
movq %rbp, %rsp # Reset stack to base pointer.
popq %rbp # Restore the old base pointer
ret # Return to caller
And here is my C code:
extern int dec(int num, char* c);
#include <stdio.h>
int main(){
char* a = "Test\n";
dec(0x100, a);
printf("Num: %s\n", a);
}
It compiles without any problems, but when I try to run, it segfaults.
I've tried debugging it with gdb, and apparently the problem occurs when I try to run the instruction
pop (%rsi)
So, I made a few changes in my C code:
extern int dec(int num, char* c);
#include <stdio.h>
int main(){
char c;
dec(0x100, &c);
printf("Num: %s\n", &c);
}
Now, when I attempt to run it, I get this message:
Num: 256
*** stack smashing detected ***: ./teste.out terminated
Aborted (core dumped)
Can someone help me understand what's going on here and how do I fix my code?
Thanks in advance.