Fixed SDL_GetIOSize() return value

Fixes https://github.com/libsdl-org/SDL/issues/15129

(cherry picked from commit 0a1c7b5348396f37998d4220332ee220748b6b01)

src/io/SDL_iostream.c

@@ -1404,7 +1404,8 @@ SDL_PropertiesID SDL_GetIOProperties(SDL_IOStream *context)
 Sint64 SDL_GetIOSize(SDL_IOStream *context)
 {
     CHECK_PARAM(!context) {
-        return SDL_InvalidParamError("context");
+        SDL_InvalidParamError("context");
+        return -1;
     }
 
     if (!context->iface.size) {