lpc: use default number of passes when lpc_passes is invalid

Should fix valgrind failure (uninitialized vars)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/lpc.c

@@ -192,6 +192,9 @@ int ff_lpc_calc_coefs(LPCContext *s,
         LLSModel m[2];
         double var[MAX_LPC_ORDER+1], av_uninit(weight);
 
+        if(lpc_passes <= 0)
+            lpc_passes = 2;
+
         for(pass=0; pass<lpc_passes; pass++){
             av_init_lls(&m[pass&1], max_order);