build: Make -Werror=int-conversion fatal

This should hopefully avoid me accidentally passing NULL for an `int`
file descriptor in the future.

Closes: #431
Approved by: giuseppe

configure.ac

@@ -27,6 +27,7 @@ CC_CHECK_FLAGS_APPEND([WARN_CFLAGS], [CFLAGS], [\
         -Werror=missing-declarations \
         -Werror=return-type \
         -Werror=overflow \
+        -Werror=int-conversion \
 	-Werror=missing-include-dirs -Werror=aggregate-return \
 	-Werror=declaration-after-statement \
 ])