Fix crash when loading a core with unexpected register section size

Fix crash when loading a core with unexpected register section size

When loading a core without an executable like so:
 $ gdb --core core
for example often the gdbarch won't contain the
iterate_over_regset_sections method. For example on ARM.

This will generate a call to get_core_register_section with a NULL regset
like at corelow.c:628

get_core_register_section (regcache, NULL, ".reg", 0, 0, "general-purpose", 1);

However a check for REGSET_VARIABLE_SIZE in get_core_register_section
assumes that regset is != NULL thus leading to a crash with this backtrace:

(gdb) bt
#0  0x000000000065907b in get_core_register_section
    (regcache=regcache@entry=0x2c26260, regset=regset@entry=0x0,
    name=name@entry=0xdbf7b2 ".reg", min_size=min_size@entry=0,
    which=which@entry=0, human_name=human_name@entry=0xdbac28
    "general-purpose", required=1)
    at ../../gdb/corelow.c:542
#1  0x0000000000659b70 in get_core_registers (ops=<optimized out>,
    regcache=0x2c26260, regno=<optimized out>) at ../../gdb/corelow.c:628
#2  0x000000000076e5fb in target_fetch_registers
    (regcache=regcache@entry=0x2c26260, regno=regno@entry=15)
    at ../../gdb/target.c:3590

Note that commit: f962539ad23759 ("Warn if core file register
section is larger than expected") introduced this issue.
Thus releases > 7.8.2 are affected.

Also, this would have been caught by gdb.base/corefile.exp but the
problem is that this triggers only if the core dump is missing some data
so that it's not recognized as a linux core dump, or it's not a linux core
dump and the core file register section is larger than expected.

So if you just create a core and read it on linux with ARM the osabi is
detected properly and iterate_over_regset_sections is present and so the
problem is not triggered.

Thus creating a linux test for this with a crafted core that meets the
problem requirements is non-trivial.

This patch fixes this crash by adding a check for regset existence before
running the condition.

gdb/ChangeLog:

	* corelow.c (get_core_register_section): Check for regset
	existence before checking for REGSET_VARIABLE_SIZE.