[C and C++] && Operator precedence

I would like to know the answer to this for both C and C++ as I use both 
languages (usually in differen projects).

Assume the code
if( A && B )

Normally A is executed before B and this is how I have always believed 
it is guaranteed to work. But recently I got second hand information 
that the execution order is not quaranteed. E.g. B could be executed 
before A by some compilers.

Different execution order could cause e.g. application to crash if we 
first test is the pointer valid and then call a function for that pointer:
if( p && p->func() )


So. Is the execution order guaranteed by the standard in both C and C++ 
to be first A and then B or can it change e.g. because of optimization? 
Should I start writing my code in this format to be safe?

if( p )
   if( p->func() )
     ...