[autoconf] Fixed .git submodule detection test.

[dsokoloski]

The .git submodule detection test does not check that the path within the parent project actually exists. The result is that the NDPI_API_VERSION define can be left empty and subsequent compilation will fail with:

ndpi_main.c: In function 'ndpi_get_api_version':
ndpi_main.c:7641:26: error: expected expression before ')' token
   return(NDPI_API_VERSION);
                          ^
ndpi_main.c:7642:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

This PR adds a check to ensure the parent's path exists and is a directory.

This was discovered while building for OpenWrt.

Signed-off-by: Darryl Sokoloski [email protected]

[sonarcloud]

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