fix vulnerability alert II.

- use size_t for audioLength to fix the alert
- use unsigned type to make the code more robust

fixes #13

3rdParty/SDL2_mixer/src/mixer.c

@@ -829,7 +829,7 @@ Mix_Audio* Mix_LoadWAV_RW(Mix_RWops* src, SDL_bool stream)
 #ifdef FULL // SELF_CONV
     SDL_AudioCVT wavecvt;
 #endif
-    int audioLength;
+    size_t audioLength;
 #ifdef FULL // WAV_CHECK
     /* rcg06012001 Make sure src is valid */
     if (!src) {

3rdParty/SDL2_mixer/src/types_internal.h

@@ -267,8 +267,8 @@ typedef struct {
     Sint64 samplesize;
 #endif
 #else
-    int start;
-    int stop;
+    unsigned start;
+    unsigned stop;
 #ifdef FULL // MUS_ENC, SEEK
     int samplesize;
 #endif
@@ -323,7 +323,7 @@ typedef struct _Mix_Audio {
     int lastChannel;
 #ifndef FULL // SELF_CONV
     void (*converters[3])(Mix_BuffOps* buf);
-    int convMpl;
+    unsigned convMpl;
 #endif
 } _Mix_Audio;
 #endif // FULL - FIX_MUS