Build with -Werror=return-type

[jengelh]

When -Werror=return-type is used, the following problem spots are identified:

cc  -Werror=return-type -O2 -c   ./src/cuprstab.c 
./src/cuprstab.c: In function ‘uPartnStabRefine’:
./src/cuprstab.c:304:1: error: no return statement in function returning non-void [-Werror=return-type]
  304 | }
      | ^
./src/cuprstab.c: In function ‘PaStbR’:
./src/cuprstab.c:495:1: error: no return statement in function returning non-void [-Werror=return-type]
  495 | }
      | ^
./src/cuprstab.c: In function ‘UprStb’:
./src/cuprstab.c:128:1: error: control reaches end of non-void function [-Werror=return-type]
  128 | }
      | ^
./src/cuprstab.c: In function ‘UPrImg’:
./src/cuprstab.c:186:1: error: control reaches end of non-void function [-Werror=return-type]
  186 | }
      | ^

[fingolfin]

Duplicate of #86