fix a minor issue PP20 depacker:

The PowerPacker 2.0 depacker could attempt to skip more than 32 bits in
a manner that results in invalid shifts. These files are now rejected.
The source of a decompilation project suggests that skip values greater
than 32 are impossible anyway.

Ported from libxmp/libxmp#492, patch authored by
Alice Rowan.

Author: sezero

src/mmcmp.cpp

@@ -514,6 +514,7 @@ BOOL PP20_Unpack(LPCBYTE *ppMemFile, LPDWORD pdwMemLength)
 	memcpy(tmp,&lpMemFile[dwMemLength-4],4);
 	dwDstLen = (tmp[0]<<16) | (tmp[1]<<8) | tmp[2];
 	skip = tmp[3];
+	if (skip > 32) return 0;
 
 	/* original pp20 only support efficiency
 	 * from 9 9 9 9 up to 9 10 12 13, afaik,