readelf: fix out of range subtraction, seg fault from a NULL pointer and memory exhaustion, all from parsing corrupt binaries.

	PR binutils/21379
	* readelf.c (process_dynamic_section): Detect over large section
	offsets in the DT_SYMTAB entry.

	PR binutils/21345
	* readelf.c (process_mips_specific): Catch an unfeasible memory
	allocation before it happens and print a suitable error message.