Fix undefined left shift

Small unsigned integers promote to signed int. As a consequence, if v is
a uint8_t, then (v << 24) yields an int, so the left shift is undefined
if the MSB is 1.

Cast to uint32_t to yield an unsigned value.

Reported by USAN (meson x -Db_sanitize=undefined):

    runtime error: left shift of 255 by 24 places cannot be represented
    in type 'int'

app/src/util/buffer_util.h

@@ -33,7 +33,7 @@ buffer_read16be(const uint8_t *buf) {
 
 static inline uint32_t
 buffer_read32be(const uint8_t *buf) {
-    return (buf[0] << 24) | (buf[1] << 16) | (buf[2] << 8) | buf[3];
+    return ((uint32_t) buf[0] << 24) | (buf[1] << 16) | (buf[2] << 8) | buf[3];
 }
 
 static inline uint64_t