c_api return null on null layer

[Mek101]

Adds a check to ncnn_layer_create_by_typeindex and ncnn_layer_create_by_type to return null if the index or name don't match any known layer.

This is mostly to move segmentation faults deriving from a wrong typeindex/name near the source callsite.

[codecov-commenter]

Codecov Report

Merging #4865 (99ec5cf) into master (411a098) will decrease coverage by 0.20%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##           master    #4865      +/-   ##
==========================================
- Coverage   94.89%   94.69%   -0.20%     
==========================================
  Files         779      779              
  Lines      223188   223182       -6     
==========================================
- Hits       211795   211344     -451     
- Misses      11393    11838     +445     

Impacted Files	Coverage Δ
src/c_api.cpp	40.40% <37.50%> (-0.07%)	⬇️

... and 23 files with indirect coverage changes

[Mek101]

Seems like an error in the build system?

[nihui]

Thanks for your contribution !