detect: fix bad BUG_ON pattern

detect: fix bad BUG_ON pattern

cppcheck:

src/detect-engine-uint.c:73:13: warning: Conversion of string literal "unknown mode" to bool always evaluates to true. [incorrectStringBooleanError]
            BUG_ON("unknown mode");
            ^
src/detect-engine-uint.c:328:13: warning: Conversion of string literal "unknown mode" to bool always evaluates to true. [incorrectStringBooleanError]
            BUG_ON("unknown mode");
            ^
src/detect-pcre.c:291:25: warning: Conversion of string literal "Impossible captype" to bool always evaluates to true. [incorrectStringBooleanError]
                        BUG_ON("Impossible captype");
                        ^

Bug: #5291.
(cherry picked from commit 7e2ed11a11cae89bc07e9a25425192581687993d)