fix out-of-bounds read for negative errno in _json_c_strerror

tests/test_strerror.c

@@ -9,5 +9,9 @@ int main(int argc, char **argv)
 {
 	puts(strerror(10000));
 	puts(strerror(999));
+	/* Negative values must not index before the digit table (out-of-bounds
+	 * read); INT_MIN also exercises the unsigned magnitude handling. */
+	puts(strerror(-5));
+	puts(strerror(-2147483647 - 1));
 	return 0;
 }

tests/test_strerror.expected

@@ -1,2 +1,4 @@
 ERRNO=10000
 ERRNO=999
+ERRNO=-5
+ERRNO=-2147483648