End users wish to avoid "double packing".

https://github.com/upx/upx/issues/622
	modified:   src/p_lx_elf.cpp

src/p_lx_elf.cpp

@@ -2497,6 +2497,9 @@ PackLinuxElf64::canPack()
             max_offset = UPX_MAX(max_offset, get_te64(&phdr->p_filesz) + get_te64(&phdr->p_offset));
         }
     }
+    if (canUnpack()) {
+        throwAlreadyPacked();
+    }
     // We want to compress position-independent executable (gcc -pie)
     // main programs, but compressing a shared library must be avoided
     // because the result is no longer usable.  In theory, there is no way