Making a pattern using dynamic memory allocation

Question

I was trying to make a pattern using pointers and DMA in C. I want the user to enter a number and then print some pattern and this patterns lengt will be equal to the entered number. While I was tinkering with my code I had some issues. More explanation down below the code.

This is my code:

#include <stdio.h>
#include <stdlib.h>

void makepattern(int lenght);

int main()
{
    int deger;
    scanf("%d", &deger);
    makepattern(deger);
    return 0;
}

void makepattern(int lenght)
{
    int max = (2*lenght) - 1;
    
    char *p = (char *) malloc(lenght * sizeof(char));
    int middle = lenght - 1;
    int counter = 0;
    for(int i = 0; i < lenght; i++)
    {
        counter += 1;
        
        for(int a = 0; counter > a; a++)
        {
            *(p + middle + a) = '*';
            *(p + middle - a) = '*';
        }
        
        
        
        for(int b = 0; b < max; b++)
        {
            if(*(p + b) == '*')
            {
                printf("*");
            }
            else
            {
                printf(" ");
            }
        }
        printf("\n");
    }
    free(p);
}

the pattern should be something like this:

Everthing was fine until I tried 18. How can this happen? My code just worked fine with numbers 17 or below. But it started acting strange after 17.

Here are some images.

Answer 1

@WhozCraig's answer is correct. I was trying to write an adress which was not allocated beforehand and this caused undefined behavior.

Problem solved by just changing

char *p = (char *) malloc(lenght * sizeof(char));

to

char *p = (char *) malloc(max * sizeof(char));