fix: avoid assuming struct packing

lfs_gstate_t was assumed to be a packed array of uint32_t,
but this is not always guaranteed. Access the fields directly
instead of attempting to loop over an array of uint32_t

Fixes clang tidy warnings about use of uninitialized memory
accessed.

lfs.c

@@ -404,16 +404,20 @@ struct lfs_diskoff {
 
 // operations on global state
 static inline void lfs_gstate_xor(lfs_gstate_t *a, const lfs_gstate_t *b) {
-    for (int i = 0; i < 3; i++) {
+    a->tag ^= b->tag;
-        ((uint32_t*)a)[i] ^= ((const uint32_t*)b)[i];
+    a->pair[0] ^= b->pair[0];
-    }
+    a->pair[1] ^= b->pair[1];
 }
 
 static inline bool lfs_gstate_iszero(const lfs_gstate_t *a) {
-    for (int i = 0; i < 3; i++) {
+    if (a->tag != 0) {
-        if (((uint32_t*)a)[i] != 0) {
+        return false;
-            return false;
+    }
-        }
+    if (a->pair[0] != 0) {
+        return false;
+    }
+    if (a->pair[1] != 0) {
+        return false;
     }
     return true;
 }