libFLAC/bitreader.c: Fix undefined behaviour

The function FLAC__bitreader_read_raw_int32() triggered undefined behaviour
when sign extending an unsigned value. The Stanford Grahpics bithacks page
provided an alternative that avoided UB.

src/libFLAC/bitreader.c

@@ -418,12 +418,14 @@ FLAC__bool FLAC__bitreader_read_raw_uint32(FLAC__BitReader *br, FLAC__uint32 *va
 
 FLAC__bool FLAC__bitreader_read_raw_int32(FLAC__BitReader *br, FLAC__int32 *val, unsigned bits)
 {
+	FLAC__uint32 uval, mask;
 	/* OPT: inline raw uint32 code here, or make into a macro if possible in the .h file */
-	if(!FLAC__bitreader_read_raw_uint32(br, (FLAC__uint32*)val, bits))
+	if(!FLAC__bitreader_read_raw_uint32(br, &uval, bits))
 		return false;
-	/* sign-extend: */
-	*val <<= (32-bits);
-	*val >>= (32-bits);
+	/* sign-extend *val assuming it is currently bits wide. */
+	/* From: https://graphics.stanford.edu/~seander/bithacks.html#FixedSignExtend */
+	mask = 1u << (bits - 1);
+	*val = (uval ^ mask) - mask;
 	return true;
 }