EDID: Be more cautious about finding vendor blocks.

Many old monitors zero-fill the detailed descriptors, so check for that
to avoid a useless warning like:

(WW) RADEON(0): Unknown vendor-specific block 0

hw/xfree86/ddc/interpret_edid.c

@@ -285,6 +285,8 @@ get_std_timing_section(Uchar *c, struct std_timings *r,
     }
 }
 
+static const unsigned char empty_block[18];
+
 static void
 get_dt_md_section(Uchar *c, struct edid_version *ver, 
 		  struct detailed_monitor_section *det_mon)
@@ -336,7 +338,7 @@ get_dt_md_section(Uchar *c, struct edid_version *ver,
         det_mon[i].type = DS_UNKOWN;
         break;
       }
-      if (c[3] <= 0x0F) {
+      if (c[3] <= 0x0F && memcmp(c, empty_block, sizeof(empty_block))) {
 	det_mon[i].type = DS_VENDOR + c[3];
       }
     } else {