libdrgn: mark log format as non-NULL and disable -Wformat-zero-length

In my branch for the module API (#332), I want to log an error without
any additional context. Passing an empty format string causes a
"zero-length gnu_printf format string" warning from GCC, and passing
NULL crashes in vsnprintf().

Empty format strings are totally valid, but NULL clearly isn't, so
annotate the format parameter as non-NULL and disable
-Wformat-zero-length.

Signed-off-by: Omar Sandoval <osandov@osandov.com>

libdrgn/configure.ac

@@ -113,6 +113,7 @@ AS_IF([test "x$enable_compiler_warnings" != xno],
        -Wimplicit-fallthrough dnl
        -Wmissing-prototypes dnl
        -Wvla dnl
+       -Wno-format-zero-length dnl
        ], [WARN_CFLAGS], [$compiler_flags_test])])
 AS_IF([test "x$enable_compiler_warnings" = xerror],
       [AX_APPEND_FLAG([-Werror], [WARN_CFLAGS])])

libdrgn/log.h

@@ -67,7 +67,7 @@ void drgn_log(enum drgn_log_level level, struct drgn_program *prog,
 /**
  * Log a printf-style message followed by a @ref drgn_error at the given level.
  */
-__attribute__((__format__(__printf__, 4, 5)))
+__attribute__((__format__(__printf__, 4, 5), __nonnull__(2, 4)))
 void drgn_error_log(enum drgn_log_level level, struct drgn_program *prog,
 		    struct drgn_error *err, const char *format, ...);
 /** Log a critical message followed by a @ref drgn_error. */