Guard against UB in lsb/msb

lsb(b) and msb(b) are undefined when b == 0. This can lead to subtle bugs, where
the resulting code behaves differently on different configurations:
- It can be the home grown software LSB/MSB
- It can be the compiler generated software LSB/MSB (when using compiler
  intrinsics without the right compiler flags to allow compiler to use hardware
  LSB/MSB). Which of course depends on the compiler.
- It can be hardware LSB/MSB generated by the compiler.
- Not to mention that hardware LSB/MSB can return different value on different
  hardware when b == 0.

No functional change

Resolves #610

src/bitboard.cpp

@@ -81,11 +81,13 @@ namespace {
 /// Software fall-back of lsb() and msb() for CPU lacking hardware support
 
 Square lsb(Bitboard b) {
+  assert(b);
   return BSFTable[bsf_index(b)];
 }
 
 Square msb(Bitboard b) {
 
+  assert(b);
   unsigned b32;
   int result = 0;