Explicitly compare to NULL when looking for weak_import symbols.

Changing the comparison form affects the warning messages
generated in various circumstances. See the codereview for
details.

Also, Apple docs claim that !symbol comparisons will not work.

Spotted by Matthew Vosburgh.

Review URL: http://codereview.chromium.org/214012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26569 0039d316-1c4b-4281-b951-d872f2087c98

base/debug_util_posix.cc

@@ -121,7 +121,7 @@ void DebugUtil::BreakDebugger() {
 
 StackTrace::StackTrace() {
 #if defined(OS_MACOSX) && MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_5
-  if (!backtrace) {
+  if (backtrace == NULL) {
     count_ = 0;
     return;
   }
@@ -133,7 +133,7 @@ StackTrace::StackTrace() {
 
 void StackTrace::PrintBacktrace() {
 #if defined(OS_MACOSX) && MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_5
-  if (!backtrace_symbols_fd)
+  if (backtrace_symbols_fd == NULL)
     return;
 #endif
   fflush(stderr);
@@ -142,7 +142,7 @@ void StackTrace::PrintBacktrace() {
 
 void StackTrace::OutputToStream(std::ostream* os) {
 #if defined(OS_MACOSX) && MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_5
-  if (!backtrace_symbols)
+  if (backtrace_symbols == NULL)
     return;
 #endif
   scoped_ptr_malloc<char*> trace_symbols(backtrace_symbols(trace_, count_));