lib: fix heap buffer overflow in dlt_with_filename_and_line_number

When the source filename string is longer than 255 bytes,
dlt_with_filename_and_line_number(fina, linr) overflows the heap
buffer it allocates for dlt_user.filename:

  dlt_user.filenamelen = (uint8_t)strlen(fina);          // truncates
  ...
  dlt_user.filename = malloc(dlt_user.filenamelen + 1);  // small
  strcpy(dlt_user.filename, fina);                       // full

filenamelen is uint8_t (matching the V2 wire-format field width
declared in include/dlt/dlt_user.h.in), so the cast truncates
strlen(fina) modulo 256. The subsequent malloc allocates the
truncated length + 1, but strcpy copies the entire fina including
its real terminator, writing strlen(fina) - filenamelen bytes past
the end of the heap chunk.

Reachable in practice via long compile-time __FILE__ paths
(monorepo-rooted absolute paths) and any application that forwards
a user-controlled filename through this function.

Fix: clamp strlen(fina) to UINT8_MAX before assigning filenamelen,
allocate against the clamped length, and copy with memcpy + explicit
null terminator instead of strcpy. The on-wire field still gets the
truncated length, which matches the maximum the V2 protocol can
encode.

Author: SoundMatt

src/lib/dlt_user.c

@@ -4673,18 +4673,28 @@ DltReturnValue dlt_with_filename_and_line_number(const char *fina, const int lin
 
     /* Set filename and line number */
     dlt_user.with_filename_and_line_number = 1;
-    dlt_user.filenamelen = (uint8_t)strlen(fina);
+
+    /* filenamelen is uint8_t (matching the V2 wire-format field width).
+     * Clamp the source length to UINT8_MAX before truncation so that
+     * the malloc + copy below cannot write past the end of the heap
+     * allocation when strlen(fina) > 255. */
+    size_t fina_len = strlen(fina);
+    if (fina_len > UINT8_MAX)
+        fina_len = UINT8_MAX;
+    dlt_user.filenamelen = (uint8_t)fina_len;
+
     if (dlt_user.filename != NULL) {
         free(dlt_user.filename);
         dlt_user.filename = NULL;
     }
 
-    dlt_user.filename = (char*)malloc((size_t)dlt_user.filenamelen + 1);
+    dlt_user.filename = (char*)malloc(fina_len + 1);
     if (dlt_user.filename == NULL){
         dlt_vlog(LOG_ERR, "%s Could not allocate memory for filename", __func__);
         return DLT_RETURN_ERROR;
     }
-    strcpy(dlt_user.filename, fina);
+    memcpy(dlt_user.filename, fina, fina_len);
+    dlt_user.filename[fina_len] = '\0';
 
     dlt_user.linenumber = (uint32_t)linr;
     return DLT_RETURN_OK;