esp4: remove redundant initialization of pointer esph

Pointer esph is being assigned a value that is never read, esph is
re-assigned and only read inside an if statement, hence the
initialization is redundant and can be removed.

Cleans up clang warning:
net/ipv4/esp4.c:657:21: warning: Value stored to 'esph' during
its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>