Remove unnecessary null pointer checks

[elfring]

An extra null pointer check is not needed in functions like the following.

• binaryen/src/support/mixed_arena.h

  Lines 116 to 118 in 336baa3

  	if (allocated) {
  	delete allocated;
  	}

• binaryen/src/support/mixed_arena.h

  Lines 160 to 162 in 336baa3

  	if (next.load()) {
  	delete next.load();
  	}

[kripken]

Thanks, a PR would be welcome.