aacsbr: check sample_rate before using it, fix division by 0

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/aacsbr.c

@@ -340,9 +340,6 @@ static int sbr_make_f_master(AACContext *ac, SpectralBandReplication *sbr,
     } else
         temp = 5000;
 
-    start_min = ((temp << 7) + (sbr->sample_rate >> 1)) / sbr->sample_rate;
-    stop_min  = ((temp << 8) + (sbr->sample_rate >> 1)) / sbr->sample_rate;
-
     switch (sbr->sample_rate) {
     case 16000:
         sbr_offset_ptr = sbr_offset[0];
@@ -368,6 +365,9 @@ static int sbr_make_f_master(AACContext *ac, SpectralBandReplication *sbr,
         return -1;
     }
 
+    start_min = ((temp << 7) + (sbr->sample_rate >> 1)) / sbr->sample_rate;
+    stop_min  = ((temp << 8) + (sbr->sample_rate >> 1)) / sbr->sample_rate;
+
     sbr->k[0] = start_min + sbr_offset_ptr[spectrum->bs_start_freq];
 
     if (spectrum->bs_stop_freq < 14) {