[PATCH] Fix crash when loading a core with unexpected register section size

[Antoine Tremblay <antoine.tremblay@ericsson.com>]

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.

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: f962539ad23759af4ba8f7eece1946fdc2f5087 introcuded this
issue. Thus releases > 7.8.2 are affected.

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
                   existance before checking for REGSET_VARIABLE_SIZE.
---
 gdb/corelow.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/gdb/corelow.c b/gdb/corelow.c
index a075d9e..f43f730 100644
--- a/gdb/corelow.c
+++ b/gdb/corelow.c
@@ -515,6 +515,7 @@ get_core_register_section (struct regcache *regcache,
   struct bfd_section *section;
   bfd_size_type size;
   char *contents;
+  bool variable_size_section = false;
 
   xfree (section_name);
 
@@ -534,12 +535,16 @@ get_core_register_section (struct regcache *regcache,
     }
 
   size = bfd_section_size (core_bfd, section);
+
+  variable_size_section = (regset != NULL
+			   && regset->flags & REGSET_VARIABLE_SIZE);
+
   if (size < min_size)
     {
       warning (_("Section `%s' in core file too small."), section_name);
       return;
     }
-  if (size != min_size && !(regset->flags & REGSET_VARIABLE_SIZE))
+  if (size != min_size && !variable_size_section)
     {
       warning (_("Unexpected size of section `%s' in core file."),
 	       section_name);
-- 
2.7.4