Try to avoid ambiguity when sysctl returns ENOMEM additionally

checking the returned oldlen: when ENOMEM is due to the supplied
buffer being too short the return oldlen is equal to buffer size.

Without this additional check sockstat gets stuck in loop leaking the
memory if the returned ENOMEM was due the exceeded memorylocked
limit. This is easily can be observed running `limits -l 1k sockstat'.

Submitted by:	Andrey Zonov <andrey zonov org>
MFC after:	1 week

usr.bin/sockstat/sockstat.c

@@ -296,7 +296,7 @@ gather_inet(int proto)
 				break;
 			if (errno == ENOENT)
 				goto out;
-			if (errno != ENOMEM)
+			if (errno != ENOMEM || len != bufsize)
 				err(1, "sysctlbyname()");
 			bufsize *= 2;
 		}
@@ -424,7 +424,7 @@ gather_unix(int proto)
 			len = bufsize;
 			if (sysctlbyname(varname, buf, &len, NULL, 0) == 0)
 				break;
-			if (errno != ENOMEM)
+			if (errno != ENOMEM || len != bufsize)
 				err(1, "sysctlbyname()");
 			bufsize *= 2;
 		}
@@ -476,14 +476,15 @@ gather_unix(int proto)
 static void
 getfiles(void)
 {
-	size_t len;
+	size_t len, olen;
 
-	if ((xfiles = malloc(len = sizeof *xfiles)) == NULL)
+	olen = len = sizeof *xfiles;
+	if ((xfiles = malloc(len)) == NULL)
 		err(1, "malloc()");
 	while (sysctlbyname("kern.file", xfiles, &len, 0, 0) == -1) {
-		if (errno != ENOMEM)
+		if (errno != ENOMEM || len != olen)
 			err(1, "sysctlbyname()");
-		len *= 2;
+		olen = len *= 2;
 		if ((xfiles = realloc(xfiles, len)) == NULL)
 			err(1, "realloc()");
 	}