Explicitly allow wrapping when decoding mid-side

This does not happen for valid FLAC files. For invalid files, we may
decode garbage, but that does not mean the application should crash in
debug mode. (Rust panics on overflow in debug mode.)

There were multiple overflows here, all detected by libfuzzer with
cargo-fuzz.

src/frame.rs

@@ -365,10 +365,12 @@ fn decode_mid_side(buffer: &mut [i32]) {
         let side = *snd;
 
         // Double mid first, and then correct for truncated rounding that
-        // will have occured if side is odd.
-        let mid = (mid * 2) | (side & 1);
-        let left = (mid + side) / 2;
-        let right = (mid - side) / 2;
+        // will have occured if side is odd. Note that samples are never
+        // expected to exceed 25 bits, so the wrapping multiplication does not
+        // actually wrap for valid files.
+        let mid = mid.wrapping_mul(2) | (side & 1);
+        let left = mid.wrapping_add(side) / 2;
+        let right = mid.wrapping_sub(side) / 2;
 
         *fst = left;
         *snd = right;