Fix setptr to work in an 'if' clause & improve documentation

Use do {...} while (0) instead of {...}.
Document the order of events and purpose of setptr properly.

src/utils.h

@@ -35,17 +35,16 @@
 #define NZV(ptr) \
 	((ptr) && (ptr)[0])
 
-/**
- *  Frees @a ptr (if not @c NULL), then assigns @a result to it.
- *  @a result can be an expression using the 'old' value of @a ptr.
- *  It prevents a memory leak compared with: @code ptr = func(ptr); @endcode
+/** Assigns @a result to @a ptr, then frees the old value.
+ * @a result can be an expression using the 'old' value of @a ptr.
+ * E.g. @code setptr(str, g_strndup(str, 5)); @endcode
  **/
 #define setptr(ptr, result) \
-	{\
+	do {\
 		gpointer setptr_tmp = ptr;\
 		ptr = result;\
 		g_free(setptr_tmp);\
-	}
+	} while (0)
 
 /** Duplicates a string on the stack using @c g_alloca().
  * Like glibc's @c strdupa(), but portable.