Added an assert for truthy-preserving bool conversions

This has caught enough people that an explicit assert is warranted.
How littlefs, a c99 project, should be integrated with c89 projects
is still an open question, but no one deserves to debug this sort of
undetected casting issue.

Found by johnernberg and XinStellaris

lfs.c

diff --git a/lfs.c b/lfs.c
index 26280fa..6ec8215 100644
--- a/lfs.c
+++ b/lfs.c
@@ -3905,6 +3905,12 @@
     lfs->cfg = cfg;
     int err = 0;
 
+    // check that bool is a truthy-preserving type
+    //
+    // note the most common reason for this failure is a before-c99 compiler,
+    // which littlefs currently does not support
+    LFS_ASSERT((bool)0x80000000);
+
     // validate that the lfs-cfg sizes were initiated properly before
     // performing any arithmetic logics with them
     LFS_ASSERT(lfs->cfg->read_size != 0);