get_bits: change the failure condition in init_get_bits

Too much code relies in having init_get_bits fed with a valid
buffer and set its dimension to 0.

Check for NULL buffer instead.

libavcodec/get_bits.h

@@ -375,7 +375,7 @@ static inline int init_get_bits(GetBitContext *s, const uint8_t *buffer,
     int buffer_size;
     int ret = 0;
 
-    if (bit_size > INT_MAX - 7 || bit_size <= 0) {
+    if (bit_size > INT_MAX - 7 || bit_size < 0 || !buffer) {
         buffer_size = bit_size = 0;
         buffer = NULL;
         ret = AVERROR_INVALIDDATA;