flashsv2_prime: check block before using it.

Fixes null pointer dereference

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

libavcodec/flashsv.c

@@ -126,6 +126,9 @@ static int flashsv2_prime(FlashSVContext *s, uint8_t *src, int size)
     z_stream zs;
     int zret; // Zlib return code
 
+    if (!src)
+        return AVERROR_INVALIDDATA;
+
     zs.zalloc = NULL;
     zs.zfree  = NULL;
     zs.opaque = NULL;