udev/net-id: check all snprintf return values

gcc-8 throws an error if it knows snprintf might truncate output and the
return value is ignored:
../src/udev/udev-builtin-net_id.c: In function 'dev_pci_slot':
../src/udev/udev-builtin-net_id.c:297:47: error: '%s' directive output may be truncated writing up to 255 bytes into a region of size between 0 and 4095 [-Werror=format-truncation=]
                 snprintf(str, sizeof str, "%s/%s/address", slots, dent->d_name);
                                               ^~
../src/udev/udev-builtin-net_id.c:297:17: note: 'snprintf' output between 10 and 4360 bytes into a destination of size 4096
                 snprintf(str, sizeof str, "%s/%s/address", slots, dent->d_name);
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

Let's check all return values. This actually makes the code better, because there's
no point in trying to open a file when the name has been truncated, etc.

src/basic/stdio-util.h

@@ -27,9 +27,11 @@
 
 #include "macro.h"
 
-#define xsprintf(buf, fmt, ...) \
-        assert_message_se((size_t) snprintf(buf, ELEMENTSOF(buf), fmt, __VA_ARGS__) < ELEMENTSOF(buf), "xsprintf: " #buf "[] must be big enough")
+#define snprintf_ok(buf, len, fmt, ...) \
+        ((size_t) snprintf(buf, len, fmt, __VA_ARGS__) < (len))
 
+#define xsprintf(buf, fmt, ...) \
+        assert_message_se(snprintf_ok(buf, ELEMENTSOF(buf), fmt, __VA_ARGS__), "xsprintf: " #buf "[] must be big enough")
 
 #define VA_FORMAT_ADVANCE(format, ap)                                   \
 do {                                                                    \