Avoid undefined shift exponent in f2Pow().

Bug: 146938418
Test: atest DecoderTestXheAac ; atest DecoderTestAacDrc
Change-Id: Id28608e049244968158900848c23bf7a8298083d

libFDK/include/fixpoint_math.h

diff --git a/libFDK/include/fixpoint_math.h b/libFDK/include/fixpoint_math.h
index 373eec1..51df4d7 100644
--- a/libFDK/include/fixpoint_math.h
+++ b/libFDK/include/fixpoint_math.h
@@ -775,7 +775,7 @@
 
 /**
  * \brief return (base_m * 2^base_e) ^ N
- * \param base_m mantissa of the base
+ * \param base_m mantissa of the base. Must not be negative.
  * \param base_e exponent of the base
  * \param N power to be calculated of the base
  * \param result_e pointer to a INT where the exponent of the result will be

libFDK/src/fixpoint_math.cpp

diff --git a/libFDK/src/fixpoint_math.cpp b/libFDK/src/fixpoint_math.cpp
index 6c656fa..1e26420 100644
--- a/libFDK/src/fixpoint_math.cpp
+++ b/libFDK/src/fixpoint_math.cpp
@@ -1,7 +1,7 @@
 /* -----------------------------------------------------------------------------
 Software License for The Fraunhofer FDK AAC Codec Library for Android
 
-© Copyright  1995 - 2018 Fraunhofer-Gesellschaft zur Förderung der angewandten
+© Copyright  1995 - 2020 Fraunhofer-Gesellschaft zur Förderung der angewandten
 Forschung e.V. All rights reserved.
 
  1.    INTRODUCTION
@@ -650,6 +650,12 @@
   INT ans_lg2_e, baselg2_e;
   FIXP_DBL base_lg2, ans_lg2, result;
 
+  if (base_m <= (FIXP_DBL)0) {
+    result = (FIXP_DBL)0;
+    *result_e = 0;
+    return result;
+  }
+
   /* Calc log2 of base */
   base_lg2 = fLog2(base_m, base_e, &baselg2_e);