bootchart: ask for --rel when failed to initialize graph start time

We always read system uptime before log start time. So the uptime
should be always smaller number, except it includes system suspend
time. It seems better to ask for --rel and exit() than try to be
smart and try to recovery from this situation or generate huge
messy graphs.

src/bootchart/bootchart.c

@@ -350,6 +350,14 @@ int main(int argc, char *argv[]) {
 
         log_uptime();
 
+        if (graph_start < 0.0) {
+                fprintf(stderr,
+                        "Failed to setup graph start time.\n\nThe system uptime "
+                        "probably includes time that the system was suspended. "
+                        "Use --rel to bypass this issue.\n");
+                exit (EXIT_FAILURE);
+        }
+
         has_procfs = access("/proc/vmstat", F_OK) == 0;
 
         LIST_HEAD_INIT(head);