Build: check if dynamic lib installed for symlinks

If the library isn't actually installed, `generated_bin` will be null,
causing this to panic about a missing dependency for itself; checking
for this state avoids this.

Author: InKryption

lib/std/Build/Step/Compile.zig

@@ -1880,7 +1880,8 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
     }
 
     if (compile.kind == .lib and compile.linkage != null and compile.linkage.? == .dynamic and
-        compile.version != null and std.Build.wantSharedLibSymLinks(compile.rootModuleTarget()))
+        compile.version != null and compile.generated_bin != null and
+        std.Build.wantSharedLibSymLinks(compile.rootModuleTarget()))
     {
         try doAtomicSymLinks(
             step,