do not check for NULL in FREE macro

free(3) is safe to invoke on a NULL pointer, in which case no action is
taken. This change adjusts the FREE macros to omit this unnecessary
check.

i3-input/i3-input.h

@@ -5,12 +5,10 @@
 #include <err.h>
 
 #define die(...) errx(EXIT_FAILURE, __VA_ARGS__);
-#define FREE(pointer)          \
-    do {                       \
-        if (pointer != NULL) { \
-            free(pointer);     \
-            pointer = NULL;    \
-        }                      \
+#define FREE(pointer)   \
+    do {                \
+        free(pointer);  \
+        pointer = NULL; \
     } while (0)
 
 extern xcb_window_t root;