Fix a possible NULL pointer dereference in following an OOM error

in sqlite3ExprIsInteger(). Problem found by OSS-Fuzz.

FossilOrigin-Name: 5ec655e8e817c1ed3bfb2e576745a7cef441494ad7baf1bf9f8895e98ac19c5a

manifest

@@ -1,5 +1,5 @@
-C Remove\san\sobsolete\sassert()\sin\sthe\sIN\soperator\scode\sgeneration.
-D 2017-03-12T19:39:00.634
+C Fix\sa\spossible\sNULL\spointer\sdereference\sin\sfollowing\san\sOOM\serror\nin\ssqlite3ExprIsInteger().\sProblem\sfound\sby\sOSS-Fuzz.
+D 2017-03-12T20:28:44.701
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@@ -351,7 +351,7 @@ F src/ctime.c a9984df73898c042a5cfc8f9d8e7723d02bc35c9
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-F src/expr.c 7eac40b592672a1f3e0565ac1e66fbb87218436c134d8b2460f989b550e2eb73
+F src/expr.c f12a581f342a6fd85d14c31e4fb84f16b3dd107f54d7728dddb62cebc79d7ce1
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@@ -1562,7 +1562,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
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+Z af1824315a47164e92b4aa40f4d2923c

manifest.uuid

@@ -1 +1 @@
-18bf6aca2ac86478fd12d5020f3a41cfd2bd2dc3defe2298411f79ad308a6f73
+5ec655e8e817c1ed3bfb2e576745a7cef441494ad7baf1bf9f8895e98ac19c5a

src/expr.c

@@ -1860,6 +1860,7 @@ int sqlite3ExprContainsSubquery(Expr *p){
 */
 int sqlite3ExprIsInteger(Expr *p, int *pValue){
   int rc = 0;
+  if( p==0 ) return 0;  /* Can only happen following on OOM */
 
   /* If an expression is an integer literal that fits in a signed 32-bit
   ** integer, then the EP_IntValue flag will have already been set */