libganglia: avoid slurpfile failure path on EOF

when read returns 0 (for example in EOF), this error path shouldn't be
followed or it could lead to a bogus failure message and the unlikely
possibility that an stale errno might be EINTR and therefore trigger
a busy loop.

Reported-By: Kostas Georgiou <k.georgiou@atreides.org.uk>

monitor-core/lib/file.c

@@ -122,7 +122,7 @@ slurpfile (char * filename, char **buffer, int buflen)
 
 read:
    read_len = read(fd, db, buflen);
-   if (read_len <= 0)
+   if (read_len < 0)
       {
          if (errno == EINTR)
             goto read;