util: Refactor SysErrorString logic

Deduplicate code and error checks by making sure `s` stays `nullptr`
in case of error. Return "Unknown error" instead of an empty string in
this case.

Author: laanwj

src/util/syserror.cpp

@@ -14,22 +14,21 @@
 std::string SysErrorString(int err)
 {
     char buf[256];
-    buf[0] = 0;
     /* Too bad there are three incompatible implementations of the
      * thread-safe strerror. */
-    const char *s;
+    const char *s = nullptr;
 #ifdef WIN32
-    s = buf;
-    if (strerror_s(buf, sizeof(buf), err) != 0)
-        buf[0] = 0;
+    if (strerror_s(buf, sizeof(buf), err) == 0) s = buf;
 #else
 #ifdef STRERROR_R_CHAR_P /* GNU variant can return a pointer outside the passed buffer */
     s = strerror_r(err, buf, sizeof(buf));
 #else /* POSIX variant always returns message in buffer */
-    s = buf;
-    if (strerror_r(err, buf, sizeof(buf)))
-        buf[0] = 0;
+    if (strerror_r(err, buf, sizeof(buf)) == 0) s = buf;
 #endif
 #endif
-    return strprintf("%s (%d)", s, err);
+    if (s != nullptr) {
+        return strprintf("%s (%d)", s, err);
+    } else {
+        return strprintf("Unknown error (%d)", err);
+    }
 }