assert: make disabled asserts work with bitfield conditions.

We don't care _what_ the `sizeof` results are, we just want it to vaporize
the condition's code out of the build. So we check it with a `? 1 : 0`, so it
always boils down to a `sizeof (int)`, no matter what data type the condition
actually is.

This works with bitfields, pointers, anything that can be "truthy." If it
isn't truthy, it would fail when assertions are enabled anyhow.

Fixes #15990.

(manually cherry picked from commit daf519ae05)

include/SDL_assert.h

@@ -84,11 +84,16 @@ assert can have unique static variables associated with it.
 #define SDL_LINE    __LINE__
 
 /*
-sizeof (x) makes the compiler still parse the expression even without
-assertions enabled, so the code is always checked at compile time, but
+sizeof ((x) ? 1 : 0) makes the compiler still parse the expression even
+without assertions enabled, so the code is always checked at compile time, but
 doesn't actually generate code for it, so there are no side effects or
-expensive checks at run time, just the constant size of what x WOULD be,
+expensive checks at run time, just the constant size of what int would be,
 which presumably gets optimized out as unused.
+
+(The `? 1 : 0` is so this always dithers down to `sizeof (int)`...otherwise
+you'll have problems when bitfields are used as the condition, since the
+compiler will error out on the sizeof.)
+
 This also solves the problem of...
 
     int somevalue = blah();
@@ -107,7 +112,7 @@ disable assertions.
 #endif
 
 #define SDL_disabled_assert(condition) \
-    do { (void) sizeof ((condition)); } while (SDL_NULL_WHILE_LOOP_CONDITION)
+    do { (void) sizeof ((condition) ? 1 : 0); } while (SDL_NULL_WHILE_LOOP_CONDITION)
 
 typedef enum
 {