Cleanup: avoid changing the filepath for alembic frame range calculation

The internal utility get_sequence_len would make it's filename
argument absolute so as to scan it's directory for files.

Perform this on the directory instead so the filename can be const.

source/blender/editors/io/io_alembic.c

@@ -493,7 +493,7 @@ static int cmp_frame(const void *a, const void *b)
   return 0;
 }
 
-static int get_sequence_len(char *filename, int *ofs)
+static int get_sequence_len(const char *filename, int *ofs)
 {
   int frame;
   int numdigit;
@@ -503,13 +503,15 @@ static int get_sequence_len(char *filename, int *ofs)
   }
 
   char path[FILE_MAX];
-  BLI_path_abs(filename, BKE_main_blendfile_path_from_global());
   BLI_split_dir_part(filename, path, FILE_MAX);
 
   if (path[0] == '\0') {
     /* The filename had no path, so just use the blend file path. */
     BLI_split_dir_part(BKE_main_blendfile_path_from_global(), path, FILE_MAX);
   }
+  else {
+    BLI_path_abs(path, BKE_main_blendfile_path_from_global());
+  }
 
   DIR *dir = opendir(path);
   if (dir == NULL) {