Stuck at solving leetcode Q5 using C [=34==ERROR: AddressSanitizer] - runtime-error

Question

Please excuse for my massy code. This was best I could do.
I have been solving leetcode Q5 for a while; however, I couldn't solve it. This is the closest answer I have got.
I think it's acting correctly in terms of showing outputs. But has a runtime error when
let s = "tscvrnsnnwjzkynzxwcltutcvvhdivtmcvwdiwnbmdyfdvdiseyxyiiurpnhuuufarbwalzysetxbaziuuywugfzzmhoessycogxgujmgvnncwacziyybryxjagesgcmqdryfbofwxhikuauulaqyiztkpgmelnoudvlobdsgharsdkzzuxouezcycsafvpmrzanrixubvojyeuhbcpkuuhkxdvldhdtpkdhpiejshrqpgsoslbkfyraqbmrwiykggdlkgvbvrficmiignctsxeqslhzonlfekxexpvnblrfatvetwasewpglimeqemdgdgmemvdsrzpgacpnrbmomngjpiklqgbbalzxiikacwwzbzapqmatqmexxqhssggsyzpnvvpmzngtljlrhrjbnxgpcjuokgxcbzxqhmitcxlzfehwfiwcmwfliedljghrvrahlcoiescsbupitckjfkrfhhfvdlweeeverrwfkujjdwtcwbbbbwctwdjjukfwrreveeewldvfhhfrkfjkctipubscseioclharvrhgjldeilfwmcwifwhefzlxctimhqxzbcxgkoujcpgxnbjrhrljltgnzmpvvnpzysggsshqxxemqtamqpazbzwwcakiixzlabbgqlkipjgnmombrnpcagpzrsdvmemgdgdmeqemilgpwesawtevtafrlbnvpxexkeflnozhlsqexstcngiimcifrvbvgkldggkyiwrmbqaryfkblsosgpqrhsjeiphdkptdhdlvdxkhuukpcbhueyjovbuxirnazrmpvfascyczeuoxuzzkdsrahgsdbolvduonlemgpktziyqaluuaukihxwfobfyrdqmcgsegajxyrbyyizcawcnnvgmjugxgocysseohmzzfguwyuuizabxtesyzlawbrafuuuhnpruiiyxyesidvdfydmbnwidwvcmtvidhvvctutlcwxznykzjwnnsnrvcst"
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int is_palindrome(char * s);
void get_substring(const char *str, char *substr, int start, int end);
char * longestPalindrome(char * s){
int strlength = strlen(s);
int end = strlength - 1;
int start = 0;
char substr[1000];
char *longstr = (char *)malloc(1000 * sizeof(char));
strcpy(longstr, "");
while (start <= end)
{
get_substring(s, substr, start, end);
// printf("%s\n", substr);
if (is_palindrome(substr) > strlen(longstr))
{
// printf("UPDATE : %s\n", substr);
strcpy(longstr, substr);
}
if (end == start)
{
start++;
end = strlength - 1;
}
else
{
end--;
}
}
// printf("LONG : %s", longstr);
return longstr;
}
void get_substring(const char *str, char *substr, int start, int end) {
int length = end - start + 1;
strncpy(substr, str + start, length);
substr[length] = '\0';
}
int is_palindrome(char * s){
int length = strlen(s);
int mid = length/2;
if (length % 2 == 0)
{
// printf("EVEN : %d\n", mid);
mid--;
int upperptr = mid+1;
int i = 0;
if (s[mid] != s[upperptr])
{
// printf("Returning FALSE [0]\n");
return 0;
}
while (mid-i >= 0 && upperptr+i <= length)
{
// printf("%c and %c and mid is %c\n", s[mid-i], s[upperptr+i], s[mid]);
if (s[mid-i] != s[upperptr+i])
{
// printf("Returning FALSE [1]\n");
return 0;
}
i++;
}
}
else
{
// printf("ODD: %d\n", mid+1);
int i = 0;
while (mid-i >= 0 && mid+i <= length)
{
// printf("%c and %c and mid is %c\n", s[mid-i], s[mid+i], s[mid]);
if (s[mid-i] != s[mid+i])
{
// printf("Returning FALSE [2]\n");
return 0;
}
i++;
}
}
// printf("Returning TRUE\n");
return length;
}
I had difficulties returning a local string. then found out that local string value can't be returned and has to be dynamically malloced and the pointer has to be returned.
So I fixed it (somehow, if it is right). But I reckon this is causing different - even harder - problems that I can't manage.