Check whether the iterator p == the end iterator before trying to dereference it. This is a speculative fix for a failure found on the valgrind buildbot triggered by a clang test.

llvm-svn: 217295
diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 097986d..659914a 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -212,15 +212,15 @@
 {
   StringRef::iterator p = begin;
   *dot = end;
-  while (*p == '0' && p != end)
+  while (p != end && *p == '0')
     p++;
 
-  if (*p == '.') {
+  if (p != end && *p == '.') {
     *dot = p++;
 
     assert(end - begin != 1 && "Significand has no digits");
 
-    while (*p == '0' && p != end)
+    while (p != end && *p == '0')
       p++;
   }