fix segfault when cleaning up MixChunks on windows

We built the libSDL binaries several years ago, and so it was built
against an older runtime library (msvcrt.dll).
Now we build SDL itself against msvcr110.dll for example, and therefore
allocate on another heap than the free() is taking place, which happens
in libSDL_mixer itself.
See: http://stackoverflow.com/questions/23257226/sdl-mixer-mix-freechunk-crashing-on-sample-created-in-memory

src/Mixer/Channels.xs

@@ -214,8 +214,13 @@ mixchan_get_chunk(chan)
 		char* CLASS = "SDL::Mixer::MixChunk";
 	CODE:
 		Mix_Chunk *chunk = Mix_GetChunk(chan);
+#ifdef _WIN32
+		Mix_Chunk *copy  = msvcrt_malloc(sizeof(Mix_Chunk));
+		copy->abuf       = msvcrt_malloc( chunk->alen );
+#else
 		Mix_Chunk *copy  = malloc(sizeof(Mix_Chunk));
 		copy->abuf       = malloc( chunk->alen );
+#endif
 		memcpy( copy->abuf, chunk->abuf, chunk->alen );
 		copy->alen       = chunk->alen;
 		copy->volume     = chunk->volume;

src/defines.h

@@ -32,6 +32,19 @@
 #ifndef SDL_PERL_DEFINES_H
 #define SDL_PERL_DEFINES_H
 
+#ifdef _WIN32
+#include <windows.h>
+typedef void* (*malloc_t)(int size);
+malloc_t _msvcrt_malloc = NULL;
+extern malloc_t _msvcrt_malloc;
+
+void *msvcrt_malloc(int size) {
+	if (!_msvcrt_malloc)
+		_msvcrt_malloc = (malloc_t)GetProcAddress(GetModuleHandle("msvcrt"), "malloc");
+    return _msvcrt_malloc(size);
+}
+#endif
+
 #ifdef USE_THREADS
 PerlInterpreter *parent_perl = NULL;
 extern PerlInterpreter *parent_perl;