shared/condition: fix gcc5 warning

Fixes the warning below.

src/shared/condition.c: In function ‘condition_new’:
src/shared/condition.c:47:27: warning: logical not is only applied to the left hand side of comparison [-Wlogical-not-parentheses]
         assert(!parameter == (type == CONDITION_NULL));
                           ^
src/shared/macro.h:42:44: note: in definition of macro ‘_unlikely_’
 #define _unlikely_(x) (__builtin_expect(!!(x),0))
                                            ^
src/shared/macro.h:226:22: note: in expansion of macro ‘assert_se’
 #define assert(expr) assert_se(expr)
                      ^
src/shared/condition.c:47:9: note: in expansion of macro ‘assert’
         assert(!parameter == (type == CONDITION_NULL));
         ^