feat(build): enable -Wnull-dereference

We get this for free, likely fixed in #627 or with some of the
-fanalyzer fixes. Enable -Wnull-dereference whenever -Werror is enabled.

Signed-off-by: Nicholas Sielicki <nslick@amazon.com>

configure.ac

@@ -185,6 +185,8 @@ dnl warn if float is implicit promoted to double
 picky_compiler_flags="${picky_compiler_flags} -Wdouble-promotion"
 dnl warn for potential performance problem casts
 picky_compiler_flags="${picky_compiler_flags} -Wcast-align"
+dnl warn if a null dereference is detected
+picky_compiler_flags="${picky_compiler_flags} -Wnull-dereference"
 
 AC_ARG_ENABLE([picky-compiler],
    [AS_HELP_STRING([--disable-picky-compiler], [Disable adding picky compiler flags.])])