chunkers: remove dead fill() error-return handling

fill() is declared 'except 0' and always returns 1: errors propagate as
exceptions, so the 'if not self.fill(): return None' branches were
unreachable (and process() returning None would have crashed __next__
anyway). Call fill() plainly instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/borg/chunkers/buzhash.pyx

@@ -232,8 +232,7 @@ cdef class Chunker:
                 raise Exception("chunkifier byte count mismatch")
 
         while self.remaining < min_size + window_size + 1 and not self.eof:  # see assert in Chunker init
-            if not self.fill():
-                return None
+            self.fill()
 
         # Here we either are at eof...
         if self.eof:
@@ -268,8 +267,7 @@ cdef class Chunker:
             self.remaining -= did_bytes
 
             if self.remaining <= window_size:
-                if not self.fill():
-                    return None
+                self.fill()
 
         if self.remaining <= window_size:
             self.position += self.remaining

src/borg/chunkers/buzhash64.pyx

@@ -237,8 +237,7 @@ cdef class ChunkerBuzHash64:
                 raise Exception("chunkifier byte count mismatch")
 
         while self.remaining < min_size + window_size + 1 and not self.eof:  # see assert in Chunker init
-            if not self.fill():
-                return None
+            self.fill()
 
         # Here we either are at eof...
         if self.eof:
@@ -294,8 +293,7 @@ cdef class ChunkerBuzHash64:
             self.remaining -= did_bytes
 
             if self.remaining <= window_size:
-                if not self.fill():
-                    return None
+                self.fill()
 
         if self.remaining <= window_size:
             self.position += self.remaining

src/borg/chunkers/fastcdc.pyx

@@ -175,8 +175,7 @@ cdef class ChunkerFastCDC:
 
         # ensure at least min_size + 1 bytes are buffered, or we are at eof
         while self.remaining < min_size + 1 and not self.eof:
-            if not self.fill():
-                return None
+            self.fill()
 
         # at eof with only a remainder (< min_size + 1): emit it as the final chunk
         if self.eof and self.remaining < min_size + 1:
@@ -207,8 +206,7 @@ cdef class ChunkerFastCDC:
             if self.remaining == 0:
                 if self.eof:
                     break  # cut at end of data
-                if not self.fill():
-                    return None
+                self.fill()
                 if self.remaining == 0:
                     break  # buffer full -> chunk reached max_size -> forced cut
                 continue