vncv: fix snarf buffer realloc memory corruption

fix never updating p when snarf is reallocated,
resulting in memory corruption.

sys/src/cmd/vnc/wsys.c

@@ -281,6 +281,7 @@ getsnarf(int *sz)
 			*sz += c;
 			if (n == 0){
 				snarf = realloc(snarf, *sz + 8192);
+				p = snarf + *sz;
 				n = 8192;
 			}
 		}