Boot loader VFS: fix string termination error.

The code was resetting the pointer to NULL, instead of setting the
pointed char to null-terminate the string. The result was resolving the
current directory later on, instead of the parent as requested.

This only happened when calling open_from with O_CREAT, and a name with
sub-directories inside it. The boot loader never does that, so the code
wasn't used until now.

Fixes #12941.

Thanks to mt for spotting the problem.

src/system/boot/loader/vfs.cpp

@@ -1044,7 +1044,7 @@ open_from(Directory *directory, const char *name, int mode, mode_t permissions)
 			if (lastSlash[1] == '\0')
 				return B_ENTRY_NOT_FOUND;
 
-			lastSlash = '\0';
+			*lastSlash = '\0';
 			name = lastSlash + 1;
 
 			// resolve the directory