bfsoundlib: Allocate extra buffer only if XMI has more songs

Extra buffer is required to separate one song from XMI file.
If the file has just one song, it is not required.

Author: mefistotelis

bfsoundlib/src/mssxmidi.c

@@ -1667,9 +1667,10 @@ int32_t AIL2OAL_API_init_sequence(SNDSEQUENCE *seq, const void *start,  int32_t
         return 0;
     }
 
-    // Current XMIDI playback libraries have no ability to separate sequences
-    // Prepare a new image start, which contains only one sequence
+    if ((sequence_num > 0) || XMI_find_sequence(start, 1) != NULL)
     {
+        // Current XMIDI playback libraries have no ability to separate sequences
+        // Prepare a new image start, which contains only one sequence
         uint32_t bebytes;
 
         len = 8 + XMI_swap32(*(uint32_t*)(image + 4));
@@ -1694,6 +1695,11 @@ int32_t AIL2OAL_API_init_sequence(SNDSEQUENCE *seq, const void *start,  int32_t
         image_start = simg;
         image = simg + 34;
     }
+    else
+    {
+        simg = NULL;
+        image_start = start;
+    }
 
     // Locate IFF chunks within FORM XMID:
     //