shared, libsystemd-daemon: check for empty strings in strto*l conversions

strtol() and friends may set EINVAL if no conversion was performed, but
they are not required to do so. In practice they don't. We need to check
for it.

https://bugzilla.redhat.com/show_bug.cgi?id=870577

src/libsystemd-daemon/sd-daemon.c

@@ -88,7 +88,7 @@ _sd_export_ int sd_listen_fds(int unset_environment) {
                 goto finish;
         }
 
-        if (!p || *p || l <= 0) {
+        if (!p || p == e || *p || l <= 0) {
                 r = -EINVAL;
                 goto finish;
         }
@@ -112,7 +112,7 @@ _sd_export_ int sd_listen_fds(int unset_environment) {
                 goto finish;
         }
 
-        if (!p || *p) {
+        if (!p || p == e || *p) {
                 r = -EINVAL;
                 goto finish;
         }