Made G_str_to_sql \0-safe.

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@36618 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/gis/strings.c

@@ -299,7 +299,7 @@ int G_str_to_sql(char *str)
 
     count = 0;
 
-    if (!str)
+    if (!str || !*str)
 	return 0;
 
     c = str;