Fixed heap buffer overflow in files edit_line

```
==25903==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x50200004460f at pc 0x7fa23c10ec86 bp 0x7ffef7339c60 sp 0x7ffef7339408
READ of size 1 at 0x50200004460f thread T0
    #0 0x7fa23c10ec85 in __interceptor_strncmp ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:497
    #1 0x7fa23be8d09b in StringSafeCompareN /tmp/matchpolicy_poc_3369/cfengine-core/libntech/libutils/string_lib.c:254
    #2 0x7fa23be8d10f in StringEqualN /tmp/matchpolicy_poc_3369/cfengine-core/libntech/libutils/string_lib.c:268
    #3 0x560644d90e30 in MatchPolicy /tmp/matchpolicy_poc_3369/cfengine-core/cf-agent/files_editline.c:1749
 ---snip---
0x50200004460f is located 1 bytes to the left of 4-byte region [0x502000044610,0x502000044614)
allocated by thread T0 here:
 ---snip---
```

Ticket: ENT-13590
Changelog: Title
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi

cf-agent/files_editline.c

@@ -1746,7 +1746,7 @@ static bool MatchPolicy(EvalContext *ctx, const char *camel, const char *haystac
             }
             else if (opt == INSERT_MATCH_TYPE_IGNORE_TRAILING)
             {
-                if (!StringEqualN(final + final_size - 5, "\\s*", 3))
+                if (final_size >= 5 && !StringEqualN(final + final_size - 5, "\\s*", 3))
                 {
                     const size_t work_size = final_size + 3;
                     char  *work      = xcalloc(1, work_size);