Otherwise, the check is just undefined behaviour. gcc even takes out
this check because len can never be zero if len does not wrap around.

Found with -Wstrict-overflow=2

This is not changing the behavior of the code, but it is improving the
readability and helps to make the intention clearer.

One thing to take care: 'true' should never be compared against.

This reverts commit 3ea491c0.

Big thanks at mvollmer (https://sourceforge.net/p/cjson/bugs/45/)

The test for whether "c" is null is redundant.  Since this is the else case of "(!c)", and the only way "c" is changed is by the instruction assigning it "c->next" on the condition here, simply verifying that "c->next" isn't null is sufficient.

Functions like cJSON_Print do not and should not modify the object passed
to them.