use local variable for retval to avoid clobbering

[zsistla]

@lavarou 's great research and findings:
While I was looking for some information about -Wclobber and learning about zend_try/zend_catch I found these two interesting resources:
php/php-src#5151 - this basically shows that PHP team is aware of issues caused by -Wclobber and basically turn these warnings off (I don’t know if it’s good or bad that PHP is ignoring compiler warnings though; maybe they know what they’re doing)
https://github.com/php/php-src/blob/master/main/streams/userspace.c#L335-L340 - this is an example in PHP source code how zend_call_method_if_exists is called. It is a nice little trick of using an automatic variable to store user function call result. Based on this I came up with this idea: https://github.com/newrelic/newrelic-php-agent/compare/compiler_error...lavarou/fix/php-call-try-catch?expand=1. It seems to resolve the issue - I haven’t tested it past agent’s unit tests (make agent-valgrind) for PHP 8.2. Using automatic/local variable has this advantage that memory allocation is delayed until it is certain retval is good for use - i.e. zend_call_method_if_exists succeeded. Clobbering (longjmp called from within zend_call_method_if_exists which will cause this line to jump here) may invalidate the value of retval pointer and at the time it is freed here it may point to invalid memory and cause segfault. I don’t know this for certain. It would be awesome if there was a way to unit tests this code somehow and emulate happy path as well as exception path.