MINOR: standard: add function that converts signed int to a string

This function is the same as "ultoa_r", but it takes a signed value
as input.

include/common/standard.h

@@ -105,6 +105,7 @@ extern int strlcpy2(char *dst, const char *src, int size);
  */
 extern char itoa_str[][171];
 extern char *ultoa_r(unsigned long n, char *buffer, int size);
+extern char *sltoa_r(long n, char *buffer, int size);
 extern const char *ulltoh_r(unsigned long long n, char *buffer, int size);
 static inline const char *ultoa(unsigned long n)
 {

src/standard.c

@@ -406,6 +406,22 @@ char *ultoa_r(unsigned long n, char *buffer, int size)
 	return pos + 1;
 }
 
+/*
+ * This function simply returns a locally allocated string containing
+ * the ascii representation for signed number 'n' in decimal.
+ */
+char *sltoa_r(long n, char *buffer, int size)
+{
+	char *pos;
+
+	if (n >= 0)
+		return ultoa_r(n, buffer, size);
+
+	pos = ultoa_r(-n, buffer + 1, size - 1) - 1;
+	*pos = '-';
+	return pos;
+}
+
 /*
  * This function simply returns a locally allocated string containing
  * the ascii representation for number 'n' in decimal, formatted for