ffserver: fix incorrect strlcpy usage

Somewhat ironic that this "safe" interface is actually being used
unsafely here. This fixes the usage preventing potential null pointer
dereference, where the old code was doubly broken: ctime can return
NULL, and ctime can return an arbitrarily long buffer.
Reviewed-by: Mark Harris <mark.hsj@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

ffserver.c

@@ -305,15 +305,19 @@ static void ffm_set_write_index(AVFormatContext *s, int64_t pos,
     ffm->file_size = file_size;
 }
 
-static char *ctime1(char *buf2, int buf_size)
+static char *ctime1(char *buf2, size_t buf_size)
 {
     time_t ti;
     char *p;
 
     ti = time(NULL);
     p = ctime(&ti);
+    if (!p || !*p) {
+        *buf2 = '\0';
+        return buf2;
+    }
     av_strlcpy(buf2, p, buf_size);
-    p = buf2 + strlen(p) - 1;
+    p = buf2 + strlen(buf2) - 1;
     if (*p == '\n')
         *p = '\0';
     return buf2;