tweaks: avoid a clang warning about an expression being treated as NULL

When compiling with 'clang', it would say: warning: expression which
evaluates to zero treated as a null pointer constant of type 'char *'.

src/global.c

@@ -227,7 +227,7 @@ regmatch_t regmatches[10];
 int hilite_attribute = A_REVERSE;
 	/* The curses attribute we use to highlight something. */
 #ifndef DISABLE_COLOR
-char* specified_color_combo[] = {'\0'};
+char* specified_color_combo[] = {NULL};
 	/* The color combinations as specified in the rcfile. */
 #endif
 int interface_color_pair[] = {0};