DEV: coccinelle: add a test to detect unchecked strdup()

The coccinelle test "unchecked-strdup.cocci" detects various cases of
unchecked strdup().

dev/coccinelle/unchecked-strdup.cocci

@@ -0,0 +1,34 @@
+// find calls to strdup
+@call@
+expression ptr;
+position p;
+@@
+
+ptr@p = strdup(...);
+
+// find ok calls to strdup
+@ok@
+expression ptr;
+position call.p;
+@@
+
+ptr@p = strdup(...);
+... when != ptr
+(
+ (ptr == NULL || ...)
+|
+ (ptr == 0 || ...)
+|
+ (ptr != NULL || ...)
+|
+ (ptr != 0 || ...)
+)
+
+// fix bad calls to strdup
+@depends on !ok@
+expression ptr;
+position call.p;
+@@
+
+ptr@p = strdup(...);
++ if (ptr == NULL) return;