nat: Fix initial buffer size parameter for getline

According to man, lineptr must be set to null AND n to 0.

Change-Id: I36683884106b97ef697264716de13813c00da9bc

openbsc/src/osmo-bsc_nat/bsc_nat_rewrite_trie.c

@@ -169,7 +169,7 @@ struct nat_rewrite *nat_rewrite_parse(void *ctx, const char *filename)
 {
 	FILE *file;
 	char *line = NULL;
-	size_t n = 2342;
+	size_t n = 0;
 	struct nat_rewrite *res;
 
 	file = fopen(filename, "r");