buzhash chunkers: add len==0 check to avoid buffer over-read

Isn't called by the higher level code with len==0,
but adding the check nevertheless for cleaner and
less suspicious code.

src/borg/chunkers/buzhash.pyx

@@ -101,6 +101,8 @@ cdef uint32_t _buzhash(const unsigned char* data, size_t len, const uint32_t* h)
     """Calculate the buzhash of the given data."""
     cdef uint32_t i
     cdef uint32_t sum = 0, imod
+    if len == 0:
+        return 0
     for i in range(len - 1, 0, -1):
         imod = i & 0x1f
         sum ^= BARREL_SHIFT(h[data[0]], imod)

src/borg/chunkers/buzhash64.pyx

@@ -78,6 +78,8 @@ cdef uint64_t _buzhash64(const unsigned char* data, size_t len, const uint64_t*
     """Calculate the buzhash of the given data."""
     cdef uint64_t i
     cdef uint64_t sum = 0, imod
+    if len == 0:
+        return 0
     for i in range(len - 1, 0, -1):
         imod = i & 0x3f
         sum ^= BARREL_SHIFT64(h[data[0]], imod)