cleanup: Ensure that error codes are always initialised.

In this case, there was no way it would not be, but a code change down
the stack could cause a variable to become uninitialised. This avoids a
gcc warning and is more locally-correct.

other/analysis/run-gcc

@@ -8,12 +8,13 @@ g++ -O3 -o /dev/null amalgamation.cc \
   "${CPPFLAGS[@]}" \
   "${LDFLAGS[@]}" \
   -std=c++11 \
-  -pedantic \
   -fdiagnostics-color=always \
   -Wall \
   -Wextra \
-  -Wno-aggregate-return \
+  -Werror \
   -Wno-aggressive-loop-optimizations \
+  -Wno-error=null-dereference \
+  -Wno-error=type-limits \
   -Wno-float-conversion \
   -Wno-format-signedness \
   -Wno-missing-field-initializers \

toxcore/tox.c

@@ -1655,6 +1655,7 @@ bool tox_file_send_chunk(Tox *tox, uint32_t friend_number, uint32_t file_number,
     }
 
     /* can't happen */
+    SET_ERROR_PARAMETER(error, TOX_ERR_FILE_SEND_CHUNK_OK);
     return 0;
 }