Remove more superfluous if(p) checks around free(p)

This patch has been generated by the following Coccinelle semantic patch:

@@
expression E;
@@

-if(E) { free(E); }
+free(E);

Signed-off-by: Mikhail Gusarov <dottedmag@dottedmag.net>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Reviewed-by: Fernando Carrijo <fcarrijo@yahoo.com.br>
Reviewed-by: Matt Turner <mattst88@gmail.com>

glx/single2.c

@@ -272,10 +272,8 @@ char *__glXcombine_strings(const char *cext_string, const char *sext_string)
 	s2 = sext_string;
    }
    if (!combo_string || !s1) {
-	if (combo_string)
-	    free(combo_string);
-	if (s1)
-	    free(s1);
+	free(combo_string);
+	free(s1);
 	return NULL;
    }
    combo_string[0] = '\0';