dso: Check for NULL handle in apr_dso_sym

[gmbeard]

The voidlinux package for apr on musl fails the test_unload_library test in testdso.c without this NULL check. The test unloads the dso and then immediately tries to read a symbol from the unloaded dso, and assumes this fails. I found it was actually succeeding to read the symbol. Maybe musl's dlsym() function assumes you won't pass it a NULL handle, and this test is invoking some UB.

[gmbeard]

I've discovered that the reason this test fails is because dlclose is a no-op on musl, so maybe the test should be ignored on this platform instead.