Issue json-c#486: append a missing ".0" to negative double values too.

json_object.c

@@ -810,6 +810,7 @@ static int json_object_double_to_json_string_format(struct json_object* jso,
 	{
 		const char *std_format = "%.17g";
 		int format_drops_decimals = 0;
+		int looks_numeric = 0;
 
 		if (!format)
 		{
@@ -837,11 +838,15 @@ static int json_object_double_to_json_string_format(struct json_object* jso,
 		if (format == std_format || strstr(format, ".0f") == NULL)
 			format_drops_decimals = 1;
 
+		looks_numeric = /* Looks like *some* kind of number */
+		    isdigit((unsigned char)buf[0]) ||
+		    (size > 1 && buf[0] == '-' && isdigit((unsigned char)buf[1]));
+
 		if (size < (int)sizeof(buf) - 2 &&
-		    isdigit((unsigned char)buf[0]) && /* Looks like *some* kind of number */
-			!p && /* Has no decimal point */
+		    looks_numeric &&
+		    !p && /* Has no decimal point */
 		    strchr(buf, 'e') == NULL && /* Not scientific notation */
-			format_drops_decimals)
+		    format_drops_decimals)
 		{
 			// Ensure it looks like a float, even if snprintf didn't,
 			//  unless a custom format is set to omit the decimal.

tests/test_double_serializer.c

@@ -74,4 +74,8 @@ int main()
 		printf("ERROR: json_c_set_serialization_double_format() failed");
 
 	json_object_put(obj);
+
+	obj = json_object_new_double(-12.0);
+	printf("obj(-12.0).to_string(default format)=%s\n", json_object_to_json_string(obj));
+
 }

tests/test_double_serializer.expected

@@ -16,3 +16,4 @@ obj(12.0).to_string(default format)=12.0
 obj(12.0).to_string(%.0f)=12
 obj(12.0).to_string(%.0g)=1e+01
 obj(12.0).to_string(%.1g)=12.0
+obj(-12.0).to_string(default format)=-12.0