Fix several Coverity issues
This commit adrresses the following code issues found by Coverity:
CID 339785 Division by zero, in fading_channel_test.c
CID 355272 Division by zero, in awgn_channel_test.c
CID 355277 NULL ptr dereference, in awgn_channel_test.c
CID 359663 NULL ptr dereference, in delay_channel_test.c
CID 369544 NULL ptr dereference, in chest_dl_nbiot.c
CID 373326 Resource Leak, in dft_fftw.c
CID 373329 Resource Leak, in dft_fftw.c
CID 372878 Division by zero, in sliv_test.c
CID 372871 Division by zero, in dmrs_pdcch.c
CID 370622 Negative loop bound, in csr_rs.c
CID 370624 Negative loop bound, in csr_rs.c
CID 370626 Negative loop bound, in csr_rs.c
CID's 369568, 369594 NULL ptr dereference, in ch_awgn.c
CID 369540 Logically dead code, in refsignal_dl.c
CID 369608 Logically dead code, in refsignal_ul.c
CIDs 366291, 366296, 366297 Out-of-bounds access, in zc_sequence.c
CID 372209 Division by zero, in cqi.c
CID 370992 Uninitialized pointer read, in pdcch_test.c
CID 373334 Integer overflow, in ue_sync.h
CID 370993, 370995 Undefined division, in pdcch_test.c
CID 370994 Undefined division, in ssb_decode_test.c
CIDs 353368 353364 353365 359673 353366 353367
Explicit null dereferenced, in psss_file_test.c
CID 371865 Unchecked return value, in rf_uhd_imp.cc
CID 363810 Undefined division, in ldpc_rm_chain_test.c
CID 372209, 372211, 372213, 372216 Undefined modulo, in cqi.c
CID 339834 Array compared against 0, in chest_dl.c
CID 369589 Out of bounds access, in dmrs_pucch.c
CID 371681 Out of bounds access, in ue_sync_nr_test.c
CIDs 370761, 370825 Copy-paste error, in ssb.c
CID 369599 out of bounds read, in dmrs_pdcch_test.c
CID 363795 out of bounds read, in ldpc_dec_c_avx2_flood.c
CID 363805 out of bounds read, in ldpc_dec_c_avx2long_flood.c
CID 363821 out of bounds read, in ldpc_dec_c_flood.c
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