fix bug in left_trim: didn't work on strings with only one non-space character

src/cats_strings/string_helpers.c

@@ -71,7 +71,7 @@ char *left_trim(char *line)
                 if (!isspace(line[start_idx])) break;
         }
 
-        if (start_idx == len - 1) return line;
+        if (start_idx == len) return line;
 
         memmove(line, &line[start_idx], len - start_idx);
         line[len - start_idx] = '\0';