jit: check if dlfind failed when emitting a ccall (#50961)

Fixes
#50899 (comment)

Author: gbaraldi

src/jitlayers.cpp

@@ -1594,6 +1594,9 @@ struct JuliaOJIT::DLSymOptimizer {
                         auto libname = cast<ConstantDataArray>(GV->getInitializer())->getAsCString();
                         addr = lookup(libname.data(), fname.data());
                     } else {
+                        // Can happen if we fail the compile time dlfind i.e when we try a symbol that doesn't exist in libc
+                        if (dyn_cast<ConstantPointerNull>(libarg))
+                            continue;
                         assert(cast<ConstantExpr>(libarg)->getOpcode() == Instruction::IntToPtr && "libarg should be either a global variable or a integer index!");
                         libarg = cast<ConstantExpr>(libarg)->getOperand(0);
                         auto libidx = cast<ConstantInt>(libarg)->getZExtValue();

test/ccall.jl

@@ -1922,10 +1922,15 @@ function somefunction_not_found()
     ccall((:somefunction, libfrobozz), Cvoid, ())
 end
 
+function somefunction_not_found_libc()
+    ccall(:test,Int,())
+end
+
 @testset "library not found" begin
     if Sys.islinux()
         @test_throws "could not load symbol \"somefunction\"" somefunction_not_found()
     else
         @test_throws "could not load library \"\"" somefunction_not_found()
     end
+    @test_throws "could not load symbol \"test\"" somefunction_not_found_libc()
 end