Few problems in speexdec

[BojtorBogdanErvin]

1. audio_size overflows in the line 759 of speexdec.c for few minutes long audio sample if int is defined to have less than 32 bits. It can be fixed by changing its type to long (or spx_int32_t).
2. not all systems have short defined as 2 bytes. So the size(short) from line 759 is problematic when it comes to preserve the bit trueness.