Ensure that snprintf.c's fmtint() doesn't overflow when printing INT64_MIN.

This isn't actually a live bug, as the output happens to be the
same.  But it upsets tools like UBSan, which makes it worthwhile to
fix.

As it's an issue without practical consequences, don't backpatch.

Author: Andres Freund
Discussion: https://postgr.es/m/20180928001121.hhx5n6dsygqxr5wu@alap3.anarazel.de

src/port/snprintf.c

@@ -1007,6 +1007,7 @@ fmtint(long long value, char type, int forcesign, int leftjust,
 	   PrintfTarget *target)
 {
 	unsigned long long base;
+	unsigned long long uvalue;
 	int			dosign;
 	const char *cvt = "0123456789abcdef";
 	int			signvalue = 0;
@@ -1045,7 +1046,9 @@ fmtint(long long value, char type, int forcesign, int leftjust,
 
 	/* Handle +/- */
 	if (dosign && adjust_sign((value < 0), forcesign, &signvalue))
-		value = -value;
+		uvalue = -(uint64) value;
+	else
+		uvalue = (uint64) value;
 
 	/*
 	 * SUS: the result of converting 0 with an explicit precision of 0 is no
@@ -1056,8 +1059,6 @@ fmtint(long long value, char type, int forcesign, int leftjust,
 	else
 	{
 		/* make integer string */
-		unsigned long long uvalue = (unsigned long long) value;
-
 		do
 		{
 			convert[sizeof(convert) - (++vallen)] = cvt[uvalue % base];