elf: Return false if output_section is NULL

Return false if output_section is NULL so that on input

https://sourceware.org/bugzilla/attachment.cgi?id=16131

objcopy generates

objcopy: /tmp/objcopy-poc(OrcError.cpp.o): invalid entry (0x22000000) in group [3]
objcopy: /tmp/objcopy-poc(OrcError.cpp.o): invalid entry (0x21000000) in group [3]
objcopy: /tmp/objcopy-poc(OrcError.cpp.o)(.text._ZNK12_GLOBAL__N_116OrcErrorCategory7messageB5cxx11Ei): relocation 29 has invalid symbol index 1160982879
objcopy: /tmp/stv73zYw/OrcError.cpp.o[.text._ZN4llvm3orc8orcErrorENS0_12OrcErrorCodeE]: bad value

instead of

objcopy: /tmp/objcopy-poc(OrcError.cpp.o): invalid entry (0x22000000) in group [3]
objcopy: /tmp/objcopy-poc(OrcError.cpp.o): invalid entry (0x21000000) in group [3]
objcopy: /tmp/objcopy-poc(OrcError.cpp.o)(.text._ZNK12_GLOBAL__N_116OrcErrorCategory7messageB5cxx11Ei): relocation 29 has invalid symbol index 1160982879
Segmentation fault (core dumped)

	PR binutils/33075
	* elf.c (elf_map_symbols): Return false if output_section is
	NULL.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>