Fixed memory leak in 'opal_vsnprintf()'

This commit was SVN r19843.

opal/util/printf.c

@@ -275,6 +275,9 @@ int opal_vsnprintf(char *str, size_t size, const char *fmt, va_list ap)
         }
     }
 
+    /* free allocated buffer */
+    free(buf);
+
     return length;
 }