Fix unportable usage of isspace().

Must cast char argument to unsigned to avoid doing the wrong thing
with high-bit-set characters.  Oversight in commit
30b5ede715.

src/bin/pg_basebackup/pg_basebackup.c

@@ -1134,7 +1134,7 @@ escapeConnectionParameter(const char *src)
 
 	for (p = src; *p; p++)
 	{
-		if (isspace(*p))
+		if (isspace((unsigned char) *p))
 		{
 			need_quotes = true;
 			break;